Do not load incompatible cache (#875) (#1034)

A black cache created in Python 3.8 throws an unhandled
ValueError in earlier versions. This is because 3.6 does
not recognize the pickle protocol used as default in 3.8.
Accordingly, this commit:

  - Fixes read_cache to return an empty cache instead.

  - Changes the pickle protocol to 4 as the highest protocol
    fully supported by black's supported Python versions.