replace abstractclassmethod with abstract/class method

[YunchuWang]

This is to fix abc abstractclassmethod deprecation

[codecov]

Codecov Report

Merging #121 (5a13796) into dev (ff3d9aa) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #121      +/-   ##
==========================================
+ Coverage   85.61%   85.64%   +0.02%     
==========================================
  Files          47       47              
  Lines        2781     2786       +5     
  Branches      374      374              
==========================================
+ Hits         2381     2386       +5     
  Misses        324      324              
  Partials       76       76              

Flag	Coverage Δ
unittests	85.64% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
azure/functions/meta.py	78.49% <100.00%> (+0.59%)	⬆️

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