test_non_ascii_paste_text: mock call to urlopen

[blueyed]

Likely to fix flaky coverage due to requests failing sometimes.
Ref: https://codecov.io/gh/pytest-dev/pytest/compare/f7e81dab9aa8d57498511e1b5655b53cbfbed0d0...83a1f4bd668fe337d42f909cc2ef2d8d986e9748/changes

[codecov]

Codecov Report

Merging #5728 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5728      +/-   ##
==========================================
- Coverage   96.17%   96.16%   -0.01%     
==========================================
  Files         117      117              
  Lines       25864    25865       +1     
  Branches     2498     2498              
==========================================
- Hits        24875    24874       -1     
- Misses        689      690       +1     
- Partials      300      301       +1

Impacted Files	Coverage Δ
testing/test_pastebin.py	100% <100%> (ø)	⬆️
src/_pytest/pastebin.py	87.71% <0%> (-3.51%)	⬇️

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[blueyed]

@nicoddemus
I've requested your review here since the test is from you.

[nicoddemus]

Thanks!