Use pathlib.Path instead of os.path functions

[StephenBrown2]

Since httpx targets Python 3.6+, I figured it would be good to use the pathlib.Path object functions rather than os.path.*, for a slightly better interface.

[sethmlarson]

This looks good! Are there any areas in our type definitions where we need to show we accept os.Path?

[StephenBrown2]

I don't believe so, as the way I changed it it will still expect a str and initialize a Path object from it. Though I did see the netrc PRs come through that use os.path, so I'll rebase and update those as well, and make sure to check the types for that also.

[StephenBrown2]

Rebased as promised and looked over the definitions. I don't know if context.load_verify_locations will accept a Path object, so I've left that as being cast back to a string.

[sethmlarson]

Thanks @StephenBrown2! Will take a look later :)

[sethmlarson]

This looks great, thanks @StephenBrown2!