Moved CONFIG import so it does not get triggered when just importing mapper

[ml-evs]

This is to suppress the spurious warning reported in #568. Alternatively, we could consider making another level in the mapper hierarchy (ReallyBaseMapper...) that does not touch the config.

Closes #568.

[codecov]

Codecov Report

Merging #569 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #569   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files          62       62           
  Lines        3221     3222    +1     
=======================================
+ Hits         2956     2957    +1     
  Misses        265      265           

Flag	Coverage Δ
project	91.77% <100.00%> (+<0.01%)	⬆️
validator	64.64% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
optimade/server/mappers/entries.py	98.33% <100.00%> (+0.02%)	⬆️

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