Make BaseSerializer abstract

[Dreamsorcerer]

Fixes for CodeQL errors that have randomly started appearing in a PR.

[codecov]

Codecov Report

Merging #624 (f819bee) into master (1b839ab) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
- Coverage   99.66%   99.66%   -0.01%     
==========================================
  Files          36       36              
  Lines        3557     3551       -6     
==========================================
- Hits         3545     3539       -6     
  Misses         12       12              

Flag	Coverage Δ
unit	99.66% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiocache/serializers/serializers.py	95.45% <100.00%> (+0.06%)	⬆️
tests/acceptance/test_serializers.py	100.00% <100.00%> (ø)
tests/ut/test_serializers.py	100.00% <100.00%> (ø)

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[Dreamsorcerer]

Probably a breaking change for some people, so pushing to next release.