Drop support for python version 3.6

Python version 3.6 was supported until December 23-rd 2021 meaning its
end of life has expired before more than 20 days.
Dropping support for python version 3.6 will allow us to remove
OrderedDicts.

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

@@ -18,7 +18,7 @@ jobs:
       # Run regular TUF tests on each OS/Python combination, plus special tests
       # (sslib master) and linters on Linux/Python3.x only.
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
         os: [ubuntu-latest, macos-latest, windows-latest]
         toxenv: [py]
         include:

requirements.txt

@@ -26,7 +26,7 @@
 # 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 tuf-env-${v} -p python${v};
 #   python3 -m pip install pip-tools;
 #   pip-compile --no-header -o requirements-${v}.txt requirements.txt;

setup.cfg

@@ -21,7 +21,6 @@ classifiers =
   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
@@ -37,7 +36,7 @@ packages = find:
 scripts =
   tuf/scripts/repo.py
   tuf/scripts/client.py
-python_requires = ~=3.6
+python_requires = ~=3.7
 install_requires =
   requests>=2.19.1
   securesystemslib>=0.20.0