Drop Python 3.5 support

[pmrowla]

Will close #3765.
Docs PR treeverse/dvc.org/pull/1287

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Tasks:

• Drop 3.5 from PyPI supported version list
• Drop 3.5 CI builds
• Remove dvc.compat.fspath/dvc.compat.fspath_py35 in favor of os.fspath
• Remove unneeded fspath(path_info) wrappers inside os/os.path function calls

Things not addressed in this PR:

• os.path...(path_info) calls will not be replaced with PurePath equivalents (i.e. os.path.exists(path_info) still used rather than path_info.exists())

[pared]

AFAIK, since python3.6 dict keeps the order of inserted elements, so I think we could try to get rid of OrderedDict usages. I don't know about other use cases, but in case of dvc plot py3.5 was the only reason to implement yaml, csv and json specific data loaders utilizing OrderedDict. We can create separate task for that, as it might invlove some logic removal (eg mentioned specialized loaders).

[pared]

I searched a bit on dict thing:
https://docs.python.org/3.6/whatsnew/3.6.html#other-language-changes

The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon

So maybe we should leave the OrderedDict in the end.

[efiop]