These are a few assorted fixes to enable binding=True:

1. Enable binding=True on some non-performance-critical unpickling functions. These regularly occur in __reduce__ doctests, causing annoying doctest failures. In a few cases, the unpickling function can be removed completely.

2. Compile __getmetaclass__ with binding=False otherwise it won't work.

3. Resolve a cyclic import involving sage.rings.convert.mpfi by explicitly importing it.

4. In src/sage/categories/category.py, assign the helpers as staticmethod to prevent binding.

Component: cython

Author: Jeroen Demeyer

Branch/Commit: e240df3

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/25848