Adding None-check to locate_via_py.

[dhermes]

Follow-up to #53. Also

• Using single quotes instead of double quotes
• Adding Args/Returns section to docstring

[theacodes]

Seems it needs to not be a none-check.

[dhermes]

@jonparrott It was just the mocks.

In [1]: import py

In [2]: py.path.local.sysfind?
Signature: py.path.local.sysfind(name, checker=None, paths=None)
Docstring:
return a path object found by looking at the systems
underlying PATH specification. If the checker is not None
it will be invoked to filter matching paths.  If a binary
cannot be found, None is returned
Note: This is probably not working on plain win32 systems
but may work on cygwin.
File:      ~/.pyenv/versions/3.6.2/lib/python3.6/site-packages/py/_path/local.py
Type:      method

In [3]: py.path.local.sysfind('python')
Out[3]: local('/home/dhermes/.pyenv/versions/3.6.2/bin/python')