feat: replace base64.decodestring for base64.standard_b64decode

[tomplus]

Porting kubernetes-client/python-base#90 from the official library.

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files          20       20           
  Lines        1352     1352           
=======================================
  Hits         1276     1276           
  Misses         76       76

Impacted Files	Coverage Δ
kubernetes_asyncio/config/kube_config.py	92.33% <ø> (ø)	⬆️
kubernetes_asyncio/config/kube_config_test.py	93.76% <100%> (ø)	⬆️

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