Improve error message when it is not possible to determine a function's signature

[nicoddemus]

This is an improvement as it at least indicates which is the problematic function:

============================================= ERRORS ==============================================
__________________________________ ERROR collecting test-foo.py ___________________________________
Could not determine arguments of <bound method TestCase.fix of <test-foo.TestCase object at 0x000001A6AA687908>>: invalid method signature
!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
===================================== 1 error in 0.10 seconds =====================================

Fix #4026

[asottile]

🙏

[nicoddemus]

damn dude, I was just thinking about this. It's crazy that your commit is within minutes of me trying to track this issue down again. props

🤣

I just wanted to relax a bit and decided to fix some old issues, and this one appeared on my search. What a coincidence! 😁

[codecov]

Codecov Report

Merging #4196 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4196      +/-   ##
==========================================
+ Coverage   95.81%   95.83%   +0.01%     
==========================================
  Files         109      109              
  Lines       24231    24343     +112     
  Branches     2389     2416      +27     
==========================================
+ Hits        23216    23328     +112     
+ Misses        720      719       -1     
- Partials      295      296       +1

Flag	Coverage Δ
#docs	28.51% <33.33%> (+0.02%)	⬆️
#doctesting	28.51% <33.33%> (+0.02%)	⬆️
#linting	28.51% <33.33%> (+0.02%)	⬆️
#linux	95.61% <100%> (+0.02%)	⬆️
#nobyte	91.87% <100%> (ø)	⬆️
#numpy	92.91% <100%> (ø)	⬆️
#pexpect	41.28% <33.33%> (-0.04%)	⬇️
#py27	93.96% <100%> (+0.03%)	⬆️
#py34	92.28% <100%> (+0.01%)	⬆️
#py35	92.24% <100%> (-0.04%)	⬇️
#py36	93.95% <100%> (+0.01%)	⬆️
#py37	92.27% <100%> (-0.04%)	⬇️
#trial	92.91% <100%> (ø)	⬆️
#windows	94.05% <100%> (+0.01%)	⬆️
#xdist	93.78% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/compat.py	96.62% <100%> (+0.07%)	⬆️
testing/test_collection.py	99.76% <100%> (ø)	⬆️
src/_pytest/recwarn.py	97.6% <0%> (-0.5%)	⬇️
testing/test_recwarn.py	100% <0%> (+0.45%)	⬆️

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