pytester: spawn: skip without pexpect.spawn

[blueyed]

"pexpect" can be imported on Windows, but does not have "pexpect.spawn"
then.

Ref: https://github.com/pexpect/pexpect/blob/a803933ed53/pexpect/__init__.py#L73-L76

[codecov]

Codecov Report

Merging #5736 into features will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##           features    #5736      +/-   ##
============================================
- Coverage     96.26%   96.25%   -0.01%     
============================================
  Files           117      117              
  Lines         25900    25902       +2     
  Branches       2494     2495       +1     
============================================
  Hits          24932    24932              
- Misses          665      666       +1     
- Partials        303      304       +1

Impacted Files	Coverage Δ
src/_pytest/pytester.py	90.8% <0%> (-0.28%)	⬇️

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[nicoddemus]

Thanks!