[4.6] Fix pytest.mark.parametrize when the argvalue is an iterator

[asottile]

Backport of #5356

[nicoddemus]

Thanks! 💯

[codecov]

Codecov Report

Merging #5357 into 4.6-maintenance will decrease coverage by 4.9%.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           4.6-maintenance    #5357      +/-   ##
===================================================
- Coverage            95.32%   90.41%   -4.91%     
===================================================
  Files                  115      115              
  Lines                26384    26391       +7     
  Branches              2607     2607              
===================================================
- Hits                 25150    23861    -1289     
- Misses                 924     2205    +1281     
- Partials               310      325      +15

Impacted Files	Coverage Δ
testing/test_mark.py	99.39% <100%> (ø)	⬆️
src/_pytest/mark/structures.py	92.73% <100%> (+0.08%)	⬆️
testing/test_reports.py	10.49% <0%> (-89.51%)	⬇️
testing/test_runner_xunit.py	29.82% <0%> (-70.18%)	⬇️
testing/test_warnings.py	26.77% <0%> (-68.86%)	⬇️
testing/examples/test_issue519.py	33.33% <0%> (-66.67%)	⬇️
testing/test_runner.py	34.7% <0%> (-61.61%)	⬇️
testing/test_resultlog.py	53.6% <0%> (-46.4%)	⬇️
testing/test_pytester.py	55.21% <0%> (-34.09%)	⬇️
src/_pytest/reports.py	68.96% <0%> (-26.11%)	⬇️
... and 19 more

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