Update default implementation information

[shyamd]

Updates the default implementation information to point to this "official" python implementation.

[codecov]

Codecov Report

Merging #372 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #372   +/-   ##
=======================================
  Coverage   90.27%   90.27%           
=======================================
  Files          54       54           
  Lines        2386     2386           
=======================================
  Hits         2154     2154           
  Misses        232      232           

Flag	Coverage Δ
#unittests	90.27% <ø> (ø)

Impacted Files	Coverage Δ
optimade/server/config.py	100.00% <ø> (ø)

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[CasperWA]

Squash merge at will