Check deprecations and plan their removal.

[georgkrause]

Currently the following function are deprecated:

• Language.match_score() since Version 2.0 -> Remove in 3.6
• Language.region_name() since Version 2.0 -> Remove in 3.6
• Language.region since Version 2.0 -> Remove in 3.6
• Language.variant_names() since Version 3.1 -> Remove in 4.0
• tag_match_score() since Version 2.0 -> Remove in 3.6
• best_match() since Version 2.0 (Didn't had a deprecation warning until now) -> Remove in 3.7