Implement __long__

It is the same as __int__, because Python automatically coerces the output to
long().  That is also why we do not even test that the output is an instance
of long.

This fixes issue 3858.
Use SYMPY_INTS in Number.__new__ instead of (int, long)
Only define __long__ once on NumberSymbol
Remove extraneous whitespace
Merge pull request #2153 from asmeurer/__long__

Implement __long__