feat: drop Python 3.7 support

[codecov-commenter]

Codecov Report

Merging #601 (737f1a1) into master (c58e402) will increase coverage by 0.04%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
+ Coverage   87.94%   87.98%   +0.04%     
==========================================
  Files          62       62              
  Lines        5691     5653      -38     
  Branches     1177     1167      -10     
==========================================
- Hits         5005     4974      -31     
+ Misses        472      464       -8     
- Partials      214      215       +1     

Files	Coverage Δ
apischema/cache.py	81.81% <100.00%> (-0.54%)	:arrow_down:
apischema/conversions/converters.py	89.28% <100.00%> (-0.19%)	:arrow_down:
apischema/dependencies.py	83.33% <ø> (ø)
apischema/deserialization/__init__.py	99.64% <100.00%> (-0.01%)	:arrow_down:
apischema/discriminators.py	95.00% <ø> (-0.13%)	:arrow_down:
apischema/graphql/relay/connections.py	87.50% <100.00%> (-0.44%)	:arrow_down:
apischema/graphql/relay/global_identification.py	76.53% <100.00%> (-0.24%)	:arrow_down:
apischema/graphql/relay/mutations.py	67.81% <100.00%> (-0.37%)	:arrow_down:
apischema/json_schema/refs.py	89.47% <100.00%> (-0.11%)	:arrow_down:
apischema/json_schema/schema.py	86.04% <100.00%> (-0.05%)	:arrow_down:
... and 14 more