Temporarily exclude pydantic 1.8

[khaeru]

Tests began to fail today (https://github.com/khaeru/sdmx/runs/1992616989?check_suite_focus=true) because pydantic 1.8 was released and picked up.

This version includes improvements that will allow to eliminate some or all of the workarounds in sdmx.util; however, it also introduces pydantic/pydantic#2422, for which a workaround is not obvious. This PR temporarily sets a requirement for pydantic < 1.8, until that bug is addressed. A subsequent PR will remove & also adapt to the new version.

This will require a quick patch release, v2.2.1, so that other code installing sdmx1 gets a compatible version of pydantic.

[codecov]

Codecov Report

Merging #62 (3843655) into master (912dcdc) will decrease coverage by 2.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   96.94%   94.71%   -2.24%     
==========================================
  Files          53       53              
  Lines        4393     4393              
==========================================
- Hits         4259     4161      -98     
- Misses        134      232      +98     

Impacted Files	Coverage Δ
sdmx/source/wb_wdi.py	28.57% <0.00%> (-71.43%)	⬇️
sdmx/source/ilo.py	50.00% <0.00%> (-50.00%)	⬇️
sdmx/source/lsd.py	50.00% <0.00%> (-50.00%)	⬇️
sdmx/source/insee.py	57.14% <0.00%> (-42.86%)	⬇️
sdmx/source/istat.py	57.14% <0.00%> (-42.86%)	⬇️
sdmx/source/estat.py	47.50% <0.00%> (-42.50%)	⬇️
sdmx/source/sgr.py	50.00% <0.00%> (-40.00%)	⬇️
sdmx/source/wb.py	66.66% <0.00%> (-33.34%)	⬇️
sdmx/tests/test_sources.py	71.31% <0.00%> (-28.69%)	⬇️
sdmx/client.py	91.47% <0.00%> (-3.41%)	⬇️
... and 4 more

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