Fix "lexer" being used when uploading to bpaste.net

[goerz]

Backport of #5808

[codecov]

Codecov Report

Merging #5809 into 4.6-maintenance will increase coverage by 0.27%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           4.6-maintenance   #5809      +/-   ##
==================================================
+ Coverage            95.92%   96.2%   +0.27%     
==================================================
  Files                  115     115              
  Lines                26553   26553              
  Branches              2615    2615              
==================================================
+ Hits                 25472   25546      +74     
+ Misses                 755     702      -53     
+ Partials               326     305      -21

Impacted Files	Coverage Δ
src/_pytest/pastebin.py	93.84% <100%> (+3.07%)	⬆️
testing/test_pastebin.py	100% <100%> (ø)	⬆️
src/_pytest/fixtures.py	97.55% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.25% <0%> (+0.29%)	⬆️
testing/test_terminal.py	99.84% <0%> (+0.31%)	⬆️
testing/test_config.py	99.83% <0%> (+0.32%)	⬆️
testing/test_pdb.py	99.22% <0%> (+0.38%)	⬆️
src/_pytest/pytester.py	91.66% <0%> (+0.42%)	⬆️
src/_pytest/assertion/rewrite.py	95.77% <0%> (+0.48%)	⬆️
testing/python/fixtures.py	99.09% <0%> (+0.56%)	⬆️
... and 13 more

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[nicoddemus]

Thanks @goerz!