CLN: Use pandas.compat instead of sys.version_info for Python version checks
Hopefully this will make code changes related to dropping support for specific versions of Python slightly easier to identify.

Summary:
  - Replaced instances `sys.version_info` with equivalent checks from `pandas.compat`
  - Removed code blocks specific to unsupported versions of Python (<2.7, 3.0-3.4)

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There were some very specific examples that I left as-is:

https://github.com/pandas-dev/pandas/blob/c4b4a81f56205082ec7f12bf77766e3b74d27c37/pandas/tests/io/formats/test_to_csv.py#L13

Didn't change any Cython related code, as it appears that we want to avoid imports from outside `_libs`:
https://github.com/pandas-dev/pandas/blob/c4b4a81f56205082ec7f12bf77766e3b74d27c37/pandas/_libs/tslibs/parsing.pyx#L25-L26