Add python_requires to setup.py

See #1874.

Update Python version classifiers

Avoid useless None check when testing non-None containers for emptiness, e.g. for the result of a comprehension.

Remove overambitious constant folding of comprehensions when they occur in a boolean context. Never assume that they are empty, since the iteration might already have required side-effects.
Closes #1920.

Speed up some dict lookups in Py3.5+ by calling the faster "_PyDict_GetItem_KnownHash()" instead of "PyDict_GetItem()".

Discard loop variable resetting in ForFromStatNode for Python loop variables also after the range() optimisation (was previously implied).

Add a warning about range(enum) iteration since it's really not a good idea usually, so requiring an explicit cast to say "I know what I'm doing" seems best.

Enable for-in-range() optimisation also for small integer loops with a Python loop variable. We cannot always infer the loop variable as safe integer type, but dropping the loop itself into C is always faster than calling range().

Closes #1916.

Add some missing signal declarations such as SIGPWR

Considered to make it a warning, but CPython's forgiving behaviour seems unhelpful.  Can still make it a warning later.
Closed #1905.

Fix compiler crash when analysing a complex name as a type using sizeof(X) translation. Requires type analysis before the "arg_type" is set.
Closes #1908.

Prevent config lookup of "C line in traceback" option from clearing the live exception and calling setattr() with a live exception set.
Closes #1907.

Closes #1906.