build: Lower Python version requirement to 3.9

The library could work even with 3.6, but some dependencies require 3.9.  Also, even though 3.8 is still supported until
late 2024, moving to higher Python versions might even result in better security.

poetry.lock

@@ -36,6 +36,9 @@ files = [
     {file = "astroid-3.0.1.tar.gz", hash = "sha256:86b0bb7d7da0be1a7c4aedb7974e391b32d4ed89e33de6ed6902b4b15c97577e"},
 ]
 
+[package.dependencies]
+typing-extensions = {version = ">=4.0.0", markers = "python_version < \"3.11\""}
+
 [[package]]
 name = "babel"
 version = "2.13.1"
@@ -100,6 +103,8 @@ mypy-extensions = ">=0.4.3"
 packaging = ">=22.0"
 pathspec = ">=0.9.0"
 platformdirs = ">=2"
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
+typing-extensions = {version = ">=4.0.1", markers = "python_version < \"3.11\""}
 
 [package.extras]
 colorama = ["colorama (>=0.4.3)"]
@@ -131,8 +136,10 @@ files = [
 
 [package.dependencies]
 colorama = {version = "*", markers = "os_name == \"nt\""}
+importlib-metadata = {version = ">=4.6", markers = "python_version < \"3.10\""}
 packaging = ">=19.0"
 pyproject_hooks = "*"
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
 
 [package.extras]
 docs = ["furo (>=2023.08.17)", "sphinx (>=7.0,<8.0)", "sphinx-argparse-cli (>=1.5)", "sphinx-autodoc-typehints (>=1.10)", "sphinx-issues (>=3.0.0)"]
@@ -350,6 +357,7 @@ files = [
 [package.dependencies]
 build = ">=0.1"
 setuptools = "*"
+tomli = {version = "*", markers = "python_version < \"3.11\""}
 
 [package.extras]
 test = ["mock (>=3.0.0)", "pytest"]
@@ -464,6 +472,9 @@ files = [
     {file = "coverage-7.3.2.tar.gz", hash = "sha256:be32ad29341b0170e795ca590e1c07e81fc061cb5b10c74ce7203491484404ef"},
 ]
 
+[package.dependencies]
+tomli = {version = "*", optional = true, markers = "python_full_version <= \"3.11.0a6\" and extra == \"toml\""}
+
 [package.extras]
 toml = ["tomli"]
 
@@ -514,6 +525,20 @@ files = [
     {file = "docutils-0.20.1.tar.gz", hash = "sha256:f08a4e276c3a1583a86dce3e34aba3fe04d02bba2dd51ed16106244e8a923e3b"},
 ]
 
+[[package]]
+name = "exceptiongroup"
+version = "1.1.3"
+description = "Backport of PEP 654 (exception groups)"
+optional = false
+python-versions = ">=3.7"
+files = [
+    {file = "exceptiongroup-1.1.3-py3-none-any.whl", hash = "sha256:343280667a4585d195ca1cf9cef84a4e178c4b6cf2274caef9859782b567d5e3"},
+    {file = "exceptiongroup-1.1.3.tar.gz", hash = "sha256:097acd85d473d75af5bb98e41b61ff7fe35efe6675e4f9370ec6ec5126d160e9"},
+]
+
+[package.extras]
+test = ["pytest (>=6)"]
+
 [[package]]
 name = "filelock"
 version = "3.13.1"
@@ -766,6 +791,7 @@ files = [
 
 [package.dependencies]
 mypy-extensions = ">=1.0.0"
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
 typing-extensions = ">=4.1.0"
 
 [package.extras]
@@ -938,11 +964,17 @@ files = [
 [package.dependencies]
 astroid = ">=3.0.1,<=3.1.0-dev0"
 colorama = {version = ">=0.4.5", markers = "sys_platform == \"win32\""}
-dill = {version = ">=0.3.7", markers = "python_version >= \"3.12\""}
+dill = [
+    {version = ">=0.2", markers = "python_version < \"3.11\""},
+    {version = ">=0.3.7", markers = "python_version >= \"3.12\""},
+    {version = ">=0.3.6", markers = "python_version >= \"3.11\" and python_version < \"3.12\""},
+]
 isort = ">=4.2.5,<6"
 mccabe = ">=0.6,<0.8"
 platformdirs = ">=2.2.0"
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
 tomlkit = ">=0.10.1"
+typing-extensions = {version = ">=3.10.0", markers = "python_version < \"3.10\""}
 
 [package.extras]
 spelling = ["pyenchant (>=3.2,<4.0)"]
@@ -985,6 +1017,9 @@ files = [
     {file = "pyproject_hooks-1.0.0.tar.gz", hash = "sha256:f271b298b97f5955d53fb12b72c1fb1948c22c1a6b70b315c54cedaca0264ef5"},
 ]
 
+[package.dependencies]
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
+
 [[package]]
 name = "pytest"
 version = "7.4.3"
@@ -998,9 +1033,11 @@ files = [
 
 [package.dependencies]
 colorama = {version = "*", markers = "sys_platform == \"win32\""}
+exceptiongroup = {version = ">=1.0.0rc8", markers = "python_version < \"3.11\""}
 iniconfig = "*"
 packaging = "*"
 pluggy = ">=0.12,<2.0"
+tomli = {version = ">=1.0.0", markers = "python_version < \"3.11\""}
 
 [package.extras]
 testing = ["argcomplete", "attrs (>=19.2.0)", "hypothesis (>=3.56)", "mock", "nose", "pygments (>=2.7.2)", "requests", "setuptools", "xmlschema"]
@@ -1228,6 +1265,7 @@ babel = ">=2.9"
 colorama = {version = ">=0.4.5", markers = "sys_platform == \"win32\""}
 docutils = ">=0.18.1,<0.21"
 imagesize = ">=1.3"
+importlib-metadata = {version = ">=4.8", markers = "python_version < \"3.10\""}
 Jinja2 = ">=3.0"
 packaging = ">=21.0"
 Pygments = ">=2.14"
@@ -1363,6 +1401,17 @@ files = [
 [package.extras]
 tests = ["pytest", "pytest-cov"]
 
+[[package]]
+name = "tomli"
+version = "2.0.1"
+description = "A lil' TOML parser"
+optional = false
+python-versions = ">=3.7"
+files = [
+    {file = "tomli-2.0.1-py3-none-any.whl", hash = "sha256:939de3e7a6161af0c887ef91b7d41a53e7c5a1ca976325f429cb46ea9bc30ecc"},
+    {file = "tomli-2.0.1.tar.gz", hash = "sha256:de526c12914f0c550d15924c62d72abc48d6fe7364aa87328337a31007fe8a4f"},
+]
+
 [[package]]
 name = "tomlkit"
 version = "0.12.1"
@@ -1461,5 +1510,5 @@ testing = ["big-O", "jaraco.functools", "jaraco.itertools", "more-itertools", "p
 
 [metadata]
 lock-version = "2.0"
-python-versions = "^3.12"
+python-versions = "^3.9"
-content-hash = "d369c4e8658ac40afe61eee66a6a00eb6a7ecc6ffbf5875c83f5eb7df4d1a961"
+content-hash = "69fae4847ecb272da1713aa16b4d0d782e8fde2b1aaf5baebeb8b8a7bae3d1f3"

pyproject.toml

@@ -12,7 +12,7 @@ packages = [{include = "secret_handshake"}]
 include = ["secret_handshake/py.typed"]
 
 [tool.poetry.dependencies]
-python = "^3.12"
+python = "^3.9"
 PyNaCl = "^1.5.0"
 
 [tool.poetry.group.dev.dependencies]