Drop support for python version 3.6

Python version 3.6 was supported until December 23-rd 2021
(see https://endoflife.date/python) meaning its end of life has expired
before more than 20 days.
Dropping support for python version 3.6 will allow us to make some
small cleanups.

After a quick check I saw that Warehouse target python version 3.8.2:
- their docker file: https://github.com/pypa/warehouse/blob/main/Dockerfile#L47
- https://github.com/pypa/warehouse/blob/main/.python-version
- last pr updating pr version: pypi/warehouse#7828
Pip supports python version 3.7+ as well. They dropped python 3.6 a
couple of months ago:
pypa/pip#10641

This means it shouldn't cause headache to our users if we drop python
version 3.6 too.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       # Run tests on each OS/Python combination
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
         # TODO: Add windows-latest when gpg issues are solved
         os: [ubuntu-latest, macos-latest]
         toxenv: [py]

requirements.txt

@@ -16,7 +16,7 @@
 # 1. Use this script to create a pinned requirements file for each Python
 #    version
 # ```
-# for v in 3.6 3.7 3.8 3.9; do
+# for v in 3.7 3.8 3.9; do
 #   mkvirtualenv sslib-env-${v} -p python${v};
 #   pip install pip-tools;
 #   pip-compile --no-header -o requirements-${v}.txt requirements.txt;

setup.py

@@ -88,7 +88,6 @@
     'Operating System :: MacOS :: MacOS X',
     'Operating System :: Microsoft :: Windows',
     'Programming Language :: Python :: 3',
-    'Programming Language :: Python :: 3.6',
     'Programming Language :: Python :: 3.7',
     'Programming Language :: Python :: 3.8',
     'Programming Language :: Python :: 3.9',
@@ -101,7 +100,7 @@
     'Source': 'https://github.com/secure-systems-lab/securesystemslib',
     'Issues': 'https://github.com/secure-systems-lab/securesystemslib/issues',
   },
-  python_requires = "~=3.6",
+  python_requires = "~=3.7",
   extras_require = {
       'colors': ['colorama>=0.3.9'],
       'crypto': ['cryptography>=3.3.2'],

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = mypy, py36, py37, py38, py39, py310, purepy38, py38-no-gpg
+envlist = mypy, py37, py38, py39, py310, purepy38, py38-no-gpg
 skipsdist = True
 
 [testenv]