Deprecate support for Python 3.5

[hugovk]

In #8181 (comment), it looks like there's consensus for dropping Python 3.5 in pip 21.0 (January 2021).

As has been done previously, this adds a deprecation warning when running pip on Python 3.5.

It's not urgent, but this could be included in tomorrow's 20.2.3 release (#8511 (comment)), which would give two more months of warning than waiting for pip 20.3 in October.

If so, need to change the message from past to future tense, for example:

-                "Python 3.5 reached the end of its life on September "
+                "Python 3.5 will reach the end of its life on September "
                 "13th, 2020. Please upgrade your Python as Python 3.5 "
-                "is no longer maintained. pip 21.0 will drop support "
-                "for Python 3.5 in January 2021."
+                "won't be maintained after September 2020. pip 21.0 will "
+                "drop support for Python 3.5 in January 2021."

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Also updated gone_in for 2.7 (#6148 (comment)).