[python-trakt] Add deprecation notice for sync.get_watched

[glensc]

Requires (must be merged first):

• https://github.com/moogar0880/PyTrakt/pull/173

Deprecate method behaving incorrectly: https://github.com/moogar0880/PyTrakt/issues/160

➜ python -c "import trakt.sync; print(trakt.sync.get_watched())"
<string>:1: DeprecationWarning: Call to deprecated function (or staticmethod) get_watched. (This method returns watchlist, not watched list. This will be fixed in PyTrakt 4.x to return watched list)
[<TVShow> ...

[glensc]

Merged to my fork:

• https://github.com/glensc/python-pytrakt/commit/0a4902fb4f1e8dba252f9471811e60a119ceb21c