resolve error when running with python -OO

[JohannesBuchner]

running python with optimizations turned on strips away __doc__ and leads to an error:

python3 -OO -c 'import cosmolopy'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/mnt/data/daten/PostDoc2/home/Downloads/CosmoloPy3/cosmolopy/__init__.py", line 91, in <module>
    from . import constants as cc
  File "/mnt/data/daten/PostDoc2/home/Downloads/CosmoloPy3/cosmolopy/constants.py", line 88, in <module>
    __doc__ += "\n".join(sorted(doc.split("\n")))
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

This PR resolves this.

Also, there is a syntaxwarning about backslash escape, which I also addressed.