Remove Python 3.6 support

[jodal]

• Require Python 3.7, as 3.6 has reached end-of-life
• Run pyupgrade to 3.7

[codecov]

Codecov Report

Merging #159 (4c56d5e) into main (c1d5552) will not change coverage.
The diff coverage is n/a.

❗ Current head 4c56d5e differs from pull request most recent head 1fc67d0. Consider uploading reports for the commit 1fc67d0 to get more accurate results

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files          12       12           
  Lines         482      482           
=======================================
  Hits          456      456           
  Misses         26       26           

Impacted Files	Coverage Δ
src/pykka/_proxy.py	99.00% <ø> (ø)

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