Use string form for type checking until Python 3.7 dropped

[matthewfeickert]

What the if here? The string form is fine until you drop Pythons that don’t support it.

Originally posted by @henryiii in #1909 (comment)

This was in reference to

if T.TYPE_CHECKING:
    PathOrStr = T.Union[str, os.PathLike[str]]
else:
    PathOrStr = T.Union[str, "os.PathLike[str]"]

in PR #190 now in

pyhf/src/pyhf/typing.py

Lines 30 to 33 in ad1dd86

	if TYPE_CHECKING:
	PathOrStr = Union[str, os.PathLike[str]]
	else:
	PathOrStr = Union[str, "os.PathLike[str]"]

So until Python 3.7 is dropped (I think this is the version I am not very up to date with my type checking knowledge) we could instead just drop the if TYPE_CHECKING and use

#TODO: Switch to os.PathLike[str] once Python 3.7 dropped
PathOrStr = Union[str, "os.PathLike[str]"]

This would also allow for reverting PR #1937.