Hint for possible signal upon InvocationError

[ederag]

This is a follow up to issue #760, with changes outlined in #760 (comment).
An indication for the potential signal received is given, with the method suggested by @asottile in #760 (comment).

Tests have been reworked with a new dependency: pytest-mock. This mainly allowed to

• remove 1 out of 3 slow tests in test_z_cmdline.py.
• keep code coverage with negligible increase in testing time, while adding tests for different platforms.
  indeed, the signal hint is given only on posix systems.
  monkeypatch has been used to fake system: windows (os.name == 'nt') or posix (os.name == 'posix')

Should this PR be accepted, please merge squashing all commits into a single one, with message
hint for possible signal upon InvocationError + better tests.

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword in message body
• added news fragment in changelog folder

[codecov]

Codecov Report

Merging #766 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
+ Coverage   94.82%   94.84%   +0.01%     
==========================================
  Files          11       11              
  Lines        2397     2405       +8     
==========================================
+ Hits         2273     2281       +8     
  Misses        124      124

Impacted Files	Coverage Δ
tox/__init__.py	95.91% <100%> (+0.79%)	⬆️

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[ederag]

The appveyor tests failed with

>           assert expected_command_arg in call_command
E           assert 'python -c import sys; sys.exit(6)' in "'c:\\\\users\\\\appveyor\\\\appdata\\\\local\\\\temp\\\\1\\\\pytest-0\\\\test_exit_code_6_0\\\\foo\\\\.tox\\\\foo\\\\Scripts\\\\python.EXE -c import sys; sys.exit(6)'"

The command python given in tox.ini has been changed to python.EXE.
Is there a way to get the actual command ? result = cmd() does not contain this.
Meanwhile, just removing "python" from expected_command_arg.

[ederag]

@gaborbernat thanks for merging.
But I would have left some time for @asottile to review this request too. @asottile, if you find some improvements, please comment here, and I'll be ready to file another PR.

And the commit message could have been clearer:

Should this PR be accepted, please merge squashing all commits into a single one, with message
hint for possible signal upon InvocationError + better tests.

Or was it not possible with github ?

[gaborbernat]

It's a squash merge on master 👍

[ederag]

@gaborbernat Isn't it possible on github to do the equivalent of
git merge --squash -m "hint for possible signal upon InvocationError + better tests" ?