Refactor heap operations

[nvictus]

• Factor out heap operations to reduce redundancy
• Introduce an Empty exception for operations that attempt to pull from an empty pqdict, instead of raising a KeyError. We derive Empty from KeyError for subclass compatibility with Mapping.popitem and Mapping.clear which require a KeyError.
• Make internal heap nodes immutable and fast copy as in Make internal nodes immutables, and add very fast copy. #14
• Pydocstyle compliant docstrings
• Remove deprecated module minpq and maxpq in favor of classmethods.
• Refine type annotations

To-do:

• Update Dijkstra example to use pqdict.minpq classmethod instead of module function.