WIP: Add type stubs

[wiktor-k]

Currently this can be checked with the following command:

maturin develop && cargo run && maturin develop && mypy code.py

Currently it outputs the following errors:

$ mypy code.py
pysequoia.pyi:43: error: "__new__" must return a class instance (got "None")  [misc]
pysequoia.pyi:69: error: Argument 1 of "__eq__" is incompatible with supertype "builtins.object"; supertype defines the argument type as "object"  [override]
pysequoia.pyi:69: note: This violates the Liskov substitution principle
pysequoia.pyi:69: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
pysequoia.pyi:69: note: It is recommended for "__eq__" to work with arbitrary objects, for example:
pysequoia.pyi:69: note:     def __eq__(self, other: object) -> bool:
pysequoia.pyi:69: note:         if not isinstance(other, SignatureMode):
pysequoia.pyi:69: note:             return NotImplemented
pysequoia.pyi:69: note:         return <logic to compare two SignatureMode instances>
pysequoia.pyi:70: error: Argument 1 of "__ne__" is incompatible with supertype "builtins.object"; supertype defines the argument type as "object"  [override]
pysequoia.pyi:70: note: This violates the Liskov substitution principle
pysequoia.pyi:70: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
Found 3 errors in 1 file (checked 1 source file)

See: PyO3/pyo3#5137
Closes: #50