drop Python 3.6 support

[fmigneault]

The typing get_args function, which will now be used to improve some typing definitions (see references below) is not available until Python 3.8. In Python 3.7, it is still possible to use typing_extensions to resolve it, but Python 3.6 does not provide it. Deprecate this long overdue EOL version (no more security fixes since 23 Dec 2021, https://endoflife.date/python).

• abe66fe (#558)
• corresponding test error:

  weaver/store/base.py:3: in <module>
    from typing_extensions import Literal, get_args
    ImportError: cannot import name 'get_args'

[codecov[bot]]

Codecov Report

Merging #560 (c161f20) into master (bcbb2ec) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #560   +/-   ##
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  Coverage   84.46%   84.46%           
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  Files          79       79           
  Lines       17703    17703           
  Branches     2685     2685           
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  Hits        14952    14952           
  Misses       2022     2022           
  Partials      729      729