pyproject.toml: drop support for python versions 3.6, 3.7 and 3.8

Only 3.8 is not EOL, and it's just some months. Even Debian old stable
is currenly on 3.9

pyproject.toml

@@ -11,15 +11,12 @@ authors = [
 description = "postmarketOS tools for interacting with gitlab MRs"
 readme = "README.md"
 license = {text = "GPL-3.0-or-later"}
-requires-python = ">=3.6"
+requires-python = ">=3.9"
 classifiers = [
   "Development Status :: 5 - Production/Stable",
   "Intended Audience :: Developers",
   "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
   "Programming Language :: Python :: 3",
-  "Programming Language :: Python :: 3.6",
-  "Programming Language :: Python :: 3.7",
-  "Programming Language :: Python :: 3.8",
   "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",