Remove runtime dependency on typing_extensions package

[johnslavik]

A few improvements to package typing.

I removed the runtime dependency on the typing_extensions package.

See also python-poetry/poetry#9251 where I promoted that practice.

[johnslavik]

I wanted to move type aliases to the if TYPE_CHECKING branch, but that would pose some problems to those who don't use PEP 563.

[johnslavik]

@willmcgugan Let me know if you are interested in moving @overloads to if TYPE_CHECKING branches like here: https://github.com/jaraco/path/pull/224/files.

[willmcgugan]

Thanks.

What is the benefit of moving overloads inside a if TYPE_CHECKING block? Faster import?

[johnslavik]

Thanks.

What is the benefit of moving overloads inside a if TYPE_CHECKING block? Faster import?

I'd say less runtime overhead and less namespace mess (you don't have runtime access of typing-only overload and you don't create a few functions that are to be inexistent just a few moments later). For such a small library like this, it might not be necessary, but just showing that it's feasible.