modern packaging

[dr-leo]

• use flit in lieu of setuptools
• use importlib.resources in lieu of pkg_resources

[codecov-io]

Codecov Report

Merging #174 into master will decrease coverage by 2.93%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   95.79%   92.86%   -2.94%     
==========================================
  Files          40       40              
  Lines        3452     3446       -6     
==========================================
- Hits         3307     3200     -107     
- Misses        145      246     +101     

Impacted Files	Coverage Δ
pandasdmx/__init__.py	100.00% <100.00%> (+10.52%)	⬆️
pandasdmx/source/__init__.py	92.64% <100.00%> (-3.07%)	⬇️
pandasdmx/source/ilo.py	50.00% <0.00%> (-50.00%)	⬇️
pandasdmx/source/estat.py	42.50% <0.00%> (-45.00%)	⬇️
pandasdmx/source/istat.py	57.14% <0.00%> (-42.86%)	⬇️
pandasdmx/source/wb.py	66.66% <0.00%> (-33.34%)	⬇️
pandasdmx/tests/test_sources.py	72.22% <0.00%> (-26.99%)	⬇️
pandasdmx/source/sgr.py	50.00% <0.00%> (-20.00%)	⬇️
pandasdmx/reader/__init__.py	68.75% <0.00%> (-12.50%)	⬇️
pandasdmx/api.py	88.06% <0.00%> (-6.25%)	⬇️
... and 4 more

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