Fix pytest 7 warning

[alex-treebeard]

also update to pytest 7 for testing

https://github.com/treebeardtech/nbmake/issues/68

[codecov[bot]]

Codecov Report

Merging #75 (278d740) into main (966edac) will decrease coverage by 1.16%. The diff coverage is 82.35%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   92.46%   91.30%   -1.17%     
==========================================
  Files           4        4              
  Lines         146      161      +15     
==========================================
+ Hits          135      147      +12     
- Misses         11       14       +3     

Impacted Files	Coverage Δ
src/nbmake/pytest_plugin.py	91.30% <80.00%> (-3.44%)	:arrow_down:
src/nbmake/nb_run.py	92.10% <83.33%> (-1.75%)	:arrow_down: