Delete Python <= 3.5 specific checks from the code (#39879)
Summary:
Remove PY3 and PY34 checks from `torch/testing/_internal/common_utils.py`
Remove PY35 global var from `torch.jit.annotations`
Always call `try_get_real_signature` in `torch/jit/annotations.py`
Use `map` instead of `imap`, since Python-2 is no longer support, so map is always lazy.
Remove all pre Python-3.6 checks from `torch/_six.py` and `torch/_appdirs.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39879
Differential Revision: D22037811
Pulled By: malfet
fbshipit-source-id: af0c79f976569c2059d39ecb49c6b8285161734f
