Signal support for type checking via py.typed

[timhallmann]

Otherwise some type checkers (e.g. mypy) assume the project to be untyped.

[lilioid]

hmmm 🤔, there was a reason I didn't include that initially. I can't remember why right now though.
I'll look into it again

[lilioid]

Ahh yes, I have look at the documentation and found the following requirement:

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing. This marker applies recursively: if a top-level package includes it, all its sub-packages MUST support type checking as well.

I am not sure if all our dependencies support this by now but this was why we didn't add the py.typed file initially.

[timhallmann]

I don't think that refers to dependencies but e.g. to namespace packages such as when you have a package foo that includes the separately installable sub-packages foo.bar and foo.baz.

mypy certainly doesn't mind cryptojwt, furl and requests lacking a py.typed, and pyright generally attempts to infer as much as possible, even when there aren't any type hints. Though I do not know what pyre is doing.