Use explicit environment on subprocess

[mrclary]

Resolves #1540.

Use an explicit environment for subprocess to ensure that existing environment variables are not inherited. This ensures more reliable results.

[mrclary]

Tested locally on my system, under both conda and pyenv environments, TestSetupReadline.test_import has one failure due to assert len(difference) < 22 where len(difference) is 47 for the conda environment and 45 for th pyenv environment. Both environments have only parso==0.7.0 and pytest explicitly installed (Python 3.7.7, MacOS 10.14.6).

I'm not sure what purpose this assertion serves, since you've increased it recently from 21...
Nevertheless, all other tests pass locally for me.

Note that I do not know what effects there will be on Windows systems. In the subproccess.Popen documentation, I found:

Note: If specified, env must provide any variables required for the program to execute. On Windows, in order to run a side-by-side assembly the specified env must include a valid SystemRoot.

[davidhalter]

Thanks for this patch!

Note that I do not know what effects there will be on Windows systems

That's fine I guess, the Windows CI passes. If it doesn't work for people, I guess I will hear from them :).

Tested locally on my system, under both conda and pyenv environments, TestSetupReadline.test_import has one failure due to assert len(difference) < 22 where len(difference) is 47 for the conda environment and 45 for th pyenv environment. Both environments have only parso==0.7.0 and pytest explicitly installed (Python 3.7.7, MacOS 10.14.6).

This is test is there to ensure that the actual completions for os are not too different from typeshed's definitions. It would probably need a bit of love to make it pass on virtualenvs as well. (Just raising the numbers is not really what I want).

[davidhalter]

Minor issue: While SYSTEMROOT is probably defined most of the time, if it is not we should just not set it. The problem is essentially that it could be empty. If it is we cannot set it:

>>> os.environ['asdf'] = None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/os.py", line 730, in __setitem__
    value = self.encodevalue(value)
  File "/usr/lib/python3.5/os.py", line 798, in encode
    raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not NoneType

[mrclary]

Another issue: Windows environment variables are case insensitive, so it also be SYSTEMROOT, SystemRoot, or maybe systemroot. I'll put in some robustness and if none are present, I'll refrain from setting.

[davidhalter]

Thanks again for you work here and the good reasoning :).