Fix DeprecationWarnings due to invalid escape sequence

[hugovk]

Fixes #82

Also, please note that any invalid escape sequences in Python’s usage of the backslash in string literals now generate a DeprecationWarning and in the future this will become a SyntaxError. This behaviour will happen even if it is a valid escape sequence for a regular expression.

The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'. So r"\n" is a two-character string containing '' and 'n', while "\n" is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

https://docs.python.org/3/library/re.html

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Also, EOL Python 3.3 is no longer available on Travis CI, so I removed it and added newer 3.5-3.8 to make sure 3.x passes tests on the CI.

And it looks like Travis needs re-activating for this repo, here's this PR passing on my fork:

https://travis-ci.com/hugovk/humanize/builds/146074018

[tirkarthi]

LGTM. Thanks.

[hugovk]

Actually, this is a duplicate of #49.