Python 3.6 invalid escape sequence deprecation fixes

[Ville Skyttä]

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

[Mark M Meves (hipe)]

this issue (warnings being issued) cropped up for me when i upgraded from python 3.6 to python 3.7.

it coincided with bug 1778909, which shares the same general character of this issue.

the "fix" for me was a straightforward search-and-replace in the one file affected (`elements.py`. i probably made the same changes as @vskytta's patch has for that file).

i would be happy to help if i understood what the contributing guidelines were. i would also be happy to help migrate this whole puppy to bitbucket or github if there were interest there :P

[Leonard Richardson (leonardr)]

Resolved in revision 452. Thanks for the patch.