Make marks type safe

[flying-sheep]

• Closes N/A
• Tests included or not required because: dev process

• Release notes not necessary because: dev process

[codecov]

Codecov Report

Merging #2707 (39c3397) into master (973c4c3) will increase coverage by 0.08%.
The diff coverage is 91.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2707      +/-   ##
==========================================
+ Coverage   73.13%   73.21%   +0.08%     
==========================================
  Files         111      111              
  Lines       12220    12246      +26     
==========================================
+ Hits         8937     8966      +29     
+ Misses       3283     3280       -3     

Files	Coverage Δ
scanpy/testing/_pytest/marks.py	100.00% <100.00%> (+26.31%)	⬆️
scanpy/testing/_pytest/params.py	94.73% <ø> (ø)
scanpy/testing/_doctests.py	80.76% <60.00%> (-4.95%)	⬇️