Use mypyc to complie connection/cursor classes

[dvarrazzo]

Profiling using py-spy is showing that psycopg is now nice and fast when it can stay down in the C level, but resurfacing back in Python has a visible impact.

Rewriting the whole classes in Cython would address the problem, but it's quite a major work.

Mypyc might fill the gap, making a good use of the extensive type annotations of the psycopg codebase.

[CaselIT]

Note that cython can also compile pure python code for a moderate performance advantage. See for example falcon
I'm not sure how it compares with mypyc however wrt performance