Refactor py3

[nvictus]

• Drop py2 support
• Remove PQDict alias
• Inline type annotations and remove pyi file.
• Use backwards-compatible type machinery
• Make minpq and maxpq classmethod constructors and deprecate the functions
• Update docs
• Update tests
• Update CI

API changes:

• top: Add a default arg to return when the PQ is empty
• pop(): Support default when using pop() with PQ semantics (returns default as key when the PQ is empty).
• Add popvalue() method to mirror topvalue()