Fix context manager for LocalCommand.popen()

[yayachiken]

This fixes a regression introduced in 108342a.

It appears that this commit tried to implement a transparent proxy object for subprocess.Popen. Unfortunately, special method calls (e.g. of context manager methods by the with statement) generally circumvent the __getattribute__ machinery and perform direct lookup.

This leads to an exception when calling LocalCommand.popen() within a with statement:

>>> from plumbum import local
>>> ls = local['ls']
>>> with ls.popen():
...     print("yay")
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __enter__

This broke some code of mine. I have not found any documentation regarding this change, so I assume dropping the context manager functionality was an accident.

This PR restores the context manager functionality for local[...].popen() calls.

[henryiii]

Looks good if CI passes. Thank you!

[coveralls]

Coverage increased (+0.2%) to 82.842% when pulling 52ad039 on yayachiken:master into aed69a4 on tomerfiliba:master.

[henryiii]

Can you stick a Python 3 guard here, please?

Also, this can just be a function, rather than a class in a function (up to you).

[henryiii]

Sorry I have to ask for Python 3 guards in 2020, but plumbum still supports Python 2.6. :/ But hey, we might be able to drop Python 2.6 after November 2020 (RHEL 6)!

[yayachiken]

It it okay like this? Or do you want me to put a run-time check on the actual methods?

(I don't know what would be appropriate behaviour then, I guess just raising NotImplemented instead of AttributeError?)

[henryiii]

This is fine, adding features to Python 2 is not needed, it was an error before too, I believe.

[henryiii]

Thanks!

[yayachiken]

Thank you, that was quick!

[henryiii]

Adding tests speeds up the process considerably. ;)