Switch to ParamSpec in behave.fixture stub

Closes #1

See merge request !3

This was made possible by the recent support of PEP-612 added to mypy,
and a relevant fix for it.

Add async process interfaces

Closes #8

See merge request !1

Combine develop with main

See merge request !2

MyPy could not get a geturn type for exec_io, apparently because it
though there might be a default `deserialiser` argument?!

- Prefer constant bytes strings to constant unicode strings
- Allow both byte- & unicode- strings & path-like objects as arguments
- Pass path-like values and byte strings unchanged

Prefer passing raw byte strings to process spawner