pdb: try to import --pdbcls in pytest_configure only

[blueyed]

Fixes #5039.

[codecov]

Codecov Report

Merging #5041 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5041      +/-   ##
==========================================
+ Coverage   96.03%   96.06%   +0.03%     
==========================================
  Files         114      114              
  Lines       25757    25768      +11     
  Branches     2548     2548              
==========================================
+ Hits        24735    24754      +19     
+ Misses        711      704       -7     
+ Partials      311      310       -1

Impacted Files	Coverage Δ
src/_pytest/debugging.py	88.64% <100%> (+0.31%)	⬆️
testing/test_pdb.py	99.19% <100%> (ø)	⬆️
testing/acceptance_test.py	98.03% <0%> (+0.39%)	⬆️
src/_pytest/doctest.py	96.41% <0%> (+2.15%)	⬆️

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[JBKahn]

Thanks for getting on this 😀

[blueyed]

@JBKahn
Sure, I always like to fix regression I've caused.. ;)

Can you review / test it, please?

[nicoddemus]

Can you review / test it, please?

Oh sorry, I might have merged this too soon. 😬

[blueyed]

@nicoddemus
No worries, I've added tests.. :)

Thanks for merging!