Import ABC from collections.abc instead of collections for Python 3.9 compatibility.

[tirkarthi]

Fixes #196 . Fixes invalid escape sequences as below.

$ find . -iname '*.py'  | xargs -P 4 -I{} python3.8 -Wall -m py_compile {}

./doc/conf.py:218: DeprecationWarning: invalid escape sequence \s
  latex_elements = {'preamble': '\setcounter{tocdepth}{2}\definecolor{VerbatimBorderColor}{rgb}{1,1,1}',
./bitstring.py:2: DeprecationWarning: invalid escape sequence \ 
  """

[CAM-Gerlach]

The only builds are failing are py3.2 and py3.3, which have been EoL for many years and essentially zero current use (they hardly ever had any to begin with), due to not being able to curl the packages to install them (again, presumably due to being long obsolete). Ergo, those builds should be dropped from the Travis CI (and Trove tags and supported versions updated accordingly, possibly with a minor version bump) to allow this to be merged. I can submit a PR if you'd like, assuming this project is still maintained.

[scott-griffiths]

Minimum Python version will be 3.7, so this now solved by just using new package hierarchy. Thanks. (Sorry for the rather lengthy delay!)

[CAM-Gerlach]

Great, thanks—can't even remember why I commented on this, haha!