Removed codecov from library requirements

[crisely09]

The codecov package has been removed from PyPi as deprecated. This is causing the installing requirements to fail.

[codecov-commenter]

Codecov Report

Merging #303 (c1f5e63) into master (8251724) will decrease coverage by 0.22%. The diff coverage is 55.85%.

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
- Coverage   73.07%   72.85%   -0.22%     
==========================================
  Files          86       88       +2     
  Lines        5456     5574     +118     
==========================================
+ Hits         3987     4061      +74     
- Misses       1469     1513      +44     

Flag	Coverage Δ
unittests	72.85% <55.85%> (?)

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

Impacted Files	Coverage Δ
kgentitylinkingsklearn/entity_linking_sklearn.py	0.00% <0.00%> (ø)
..._linking/service/entity_linking_elastic_service.py	37.73% <0.00%> (-0.73%)	:arrow_down:
setup.py	0.00% <ø> (ø)
kgforge/specializations/stores/bluebrain_nexus.py	22.34% <13.33%> (-5.38%)	:arrow_down:
kgforge/specializations/resolvers/store_service.py	21.42% <13.63%> (-9.61%)	:arrow_down:
kgforge/specializations/resources/datasets.py	62.88% <18.18%> (-4.53%)	:arrow_down:
kgforge/core/archetypes/store.py	61.57% <20.00%> (ø)
kgforge/core/resource.py	76.19% <31.57%> (-13.21%)	:arrow_down:
...gforge/specializations/resolvers/agent_resolver.py	58.97% <35.71%> (+16.11%)	:arrow_up:
kgforge/core/commons/dictionaries.py	72.72% <40.00%> (-27.28%)	:arrow_down:
... and 12 more

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