ci: test with all current python versions

[ghost]

Python 3.6 is EOL and 3.10 is out since a while, make sure we test with all current versions.

[codecov[bot]]

Codecov Report

Merging #547 (8e5013c) into main (f36f6ea) will increase coverage by 0.27%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
+ Coverage   90.51%   90.79%   +0.27%     
==========================================
  Files          30       30              
  Lines        4567     4593      +26     
==========================================
+ Hits         4134     4170      +36     
+ Misses        433      423      -10     

Impacted Files	Coverage Δ
pghoard/webserver.py	89.12% <0.00%> (-0.17%)	:arrow_down:
pghoard/wal.py	100.00% <0.00%> (ø)
pghoard/basebackup/delta.py	95.86% <0.00%> (+0.01%)	:arrow_up:
pghoard/basebackup/chunks.py	96.77% <0.00%> (+0.02%)	:arrow_up:
pghoard/transfer.py	98.67% <0.00%> (+0.02%)	:arrow_up:
pghoard/archive_sync.py	86.51% <0.00%> (+0.07%)	:arrow_up:
pghoard/common.py	93.41% <0.00%> (+0.07%)	:arrow_up:
pghoard/compressor.py	93.07% <0.00%> (+0.12%)	:arrow_up:
pghoard/basebackup/base.py	85.58% <0.00%> (+0.26%)	:arrow_up:
pghoard/gnutaremu.py	82.38% <0.00%> (+0.33%)	:arrow_up:
... and 3 more