item.obj is again a bound method on TestCase function items

[nicoddemus]

Fix #5390

[codecov]

Codecov Report

Merging #5393 into master will decrease coverage by 2.72%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5393      +/-   ##
==========================================
- Coverage   92.71%   89.99%   -2.73%     
==========================================
  Files         114      114              
  Lines       25493    25654     +161     
  Branches     2480     2492      +12     
==========================================
- Hits        23637    23088     -549     
- Misses       1529     2233     +704     
- Partials      327      333       +6

Impacted Files	Coverage Δ
src/_pytest/unittest.py	94.97% <100%> (+1.18%)	⬆️
testing/test_unittest.py	99.08% <100%> (+0.02%)	⬆️
testing/test_reports.py	10.49% <0%> (-89.51%)	⬇️
testing/test_session.py	16.12% <0%> (-83.88%)	⬇️
testing/test_runner_xunit.py	24.52% <0%> (-75.48%)	⬇️
testing/test_runner.py	33.99% <0%> (-62.29%)	⬇️
testing/test_resultlog.py	52.45% <0%> (-47.55%)	⬇️
testing/test_pytester.py	54.7% <0%> (-34.76%)	⬇️
src/_pytest/reports.py	68.81% <0%> (-26.24%)	⬇️
testing/test_skipping.py	72.41% <0%> (-25.58%)	⬇️
... and 29 more

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[asottile]

[asottile]

backport: #5399