Fix extension substitution to work with *.pyc files

[Czaki]

This PR is the first to solve a problem mentioned in #38.

It is possible that call

__getattr__, __lazy_dir__, _ = lazy.attach_stub(__name__, __file__)

Will provide the file name with .pyc extension so the lazy loader tries to open .pyci file, which does not exist.

[Czaki]

@stefanv What should I do to have this reviewed/merged?

[stefanv]

This looks fine; happy to merge if the tests pass.

[Czaki]

Only linter fails on clone from gitlab. Should I fix it?

[stefanv]

No worries, I got it. Thank you!

[Czaki]

@stefanv is there a plan to release this?
I could add a second PR with documentation how to handle with pyinstaller packages that are using this package if it is required/nice.