Bump min. required Python version to 3.7

Increase it as 3.6 is EOL.

Related: https://en.wikipedia.org/wiki/History_of_Python#Table_of_versions
Reviewed-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230326164931.2390-2-ollieparanoid@postmarketos.org%3E

.ci/vermin.sh

@@ -10,7 +10,7 @@ fi
 
 # shellcheck disable=SC2046
 vermin \
-	-t=3.6- \
+	-t=3.7- \
 	--backport argparse \
 	--backport configparser \
 	--backport enum \

README.md

@@ -43,7 +43,7 @@ Issues are being tracked
   * [Windows subsystem for Linux (WSL)](https://en.wikipedia.org/wiki/Windows_Subsystem_for_Linux)
     does **not** work! Please use [VirtualBox](https://www.virtualbox.org/) instead.
   * [Linux kernel 3.17 or higher](https://postmarketos.org/oldkernel)
-* Python 3.6+
+* Python 3.7+
 * OpenSSL
 * git
 * ps

setup.py

@@ -47,15 +47,17 @@ setup(
     author_email='info@postmarketos.org',
     url='https://www.postmarketos.org',
     license='GPLv3',
-    python_requires='>=3.6',
+    python_requires='>=3.7',
     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',
     ],
     keywords='postmarketos pmbootstrap',
     packages=find_packages(exclude=['aports', 'keys', 'test']),