Issue #18441: Make test.support.requires('gui') skip when it should.
(Consolidating this check and various checks in tkinter files and moving them
to test.support and test.regrtest will be another issue.)
Issue #18441: Correct previous patch, which hg committed before I wanted it to.
Update int division to quiet 2to3 warning.
Issue #18441: add Mac (darwin) gui check. This is not needed today, but has been
in the past and might be needed again in the future
Issue #18441: Comment out code that will not compile because the standard
library has a package, lib-tk, that cannot be imported by normal means.
Lib/test/test_tk, etc, have special code to access this package.  I will not
bother with it unless the darwin check is needed before it gets moved to
test.(test-)support.py.
Issue #18441: whitespace
Issue #18441: Move commented out code to issue message.
Restore the data block size to 62.

The former block size traded away good fit within cache lines in
order to gain faster division in deque_item().  However, compilers
are getting smarter and can now replace the slow division operation
with a fast integer multiply and right shift.  Accordingly, it makes
sense to go back to a size that lets blocks neatly fill entire
cache-lines.

GCC-4.8 and CLANG 4.0 both compute "x // 62" with something
roughly equivalent to "x * 9520900167075897609 >> 69".
Update gui section of idle test README.