Drop support for Python 3.6

[lazyoracle]

What

Drop Support for Python 3.6

Why

Closes #155

How

Stop running tests and publishing releases for Python 3.6

Remarks

It should still be possible to install and use c3-toolset with Python 3.6 if one explicitly installs tensorflow<=2.6 and tensorflow-probability<=0.14 and then installs c3-toolset. However, going forward, we no longer officially support or test our builds in Python 3.6 or lower.

Checklist

Please include and complete the following checklist. Your Pull Request is (in most cases) not ready for review until the following have been completed. You can create a draft PR while you are still completing the checklist. Check the Contribution Guidelines for more details. You can mark an item as complete with the - [x] prefix

• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• Type Annotations - All new code has been type annotated in the function signatures using type hints
• Docstrings - Docstrings have been provided for functions in the numpydoc style
• Documentation - The tutorial style documentation has been updated to explain changes & new features
• Notebooks - Example notebooks have been updated to incorporate changes and new features
• Changelog - A short note on this PR has been added to the Upcoming Release section

[codecov]

Codecov Report

Merging #156 (669a215) into dev (0a91ad4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #156   +/-   ##
=======================================
  Coverage   71.41%   71.41%           
=======================================
  Files          36       36           
  Lines        5220     5220           
=======================================
  Hits         3728     3728           
  Misses       1492     1492           

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