DEP: Minimum supported Python version 3.8

[snowman2]

• Closes DEP: Python 3.7 support #930
• Fully documented, including history.rst for all changes and api/*.rst for new API

NOTE: It will have to be a "soft" pin as pypy doesn't support 3.8 yet.

[codecov]

Codecov Report

Merging #972 (3fcf577) into master (76ef185) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #972   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files          20       20           
  Lines        1752     1752           
=======================================
  Hits         1686     1686           
  Misses         66       66           

Impacted Files	Coverage Δ
pyproj/__init__.py	89.47% <ø> (ø)

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