Add clean fix for WindowsPath type when calling read_env() method

[Moon-developer]

Fix for issue #286 (WindowsPath read error)

Changes made:

• Importing WindowsPath from pathlib library in environ.py
• Isinstance check added to find WindowsPath and call "dunder string".
• implements a non-defaulted "dunder string" method (str). Casting to str() is slower.

Time difference of using "dunder string" as opposed to casting:

from timeit import timeit

timeit(lambda:str(p),number=10000000)
>>> 2.3293891000000713

timeit(lambda:p.__str__(),number=10000000)
>>> 1.3876856000000544

[xrmx]

@Moon-developer Have you tried #281 instead?

[sergeyklay]

This is resolved in develop branch. Thank you for the patch, and for helping us make django-environ better. And I am sorry about the delay.