CLN: Removing Python 3.6 or higher references that are always true
Follow up of #29212. After removing Python 3.5 compatibility, checks for >=py36 are always true and can be removed.

CC: @jreback 
mypy complaint This should be ready now, let me know if there is anything else. Thanks for the clarification @simonjayhawkins, I made the change. > This should be ready now, let me know if there is anything else.

lgtm pending https://github.com/pandas-dev/pandas/pull/29492#discussion_r344381451 Sorry, I missed that comment. Instead of updating the docstring I removed that function. With py36+ only, the function was a one liner, used just once. And it was marked as private, and used in a separate file. So, I think it was better to simply get rid of it. not sure about this, was planning to re-use https://github.com/pandas-dev/pandas/pull/25569#issuecomment-477980938 intstead of creating custom exceptions lgtm. can you merge master and @jbrockmendel has a comment, ping on green. @jreback thanks @datapythonista 
