add --notimingintensive; block from github jobs

add --notimingintensive; block from github jobs

this provides a front-end option to disable tests marked
as timing_intensive, all of which are in test_pool, which are more
fragile and aren't consistent on the
github runners.   also remove /reduce unnecessary time.sleep()
from two other pool tests that are not timing intensive.

note that this removes test_hanging_connect_within_overflow
from the github runs via the timing_intensive requirement.

I've also removed MockReconnectTest from exclusions as those are
really important tests and they use mocks so should not have
platform dependent issues.   Need to see what the
windows failures are.

Closes: #5633
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5633
Pull-request-sha: 166833e16ec342dfa10edb287d7aa495ddd1b59d

Change-Id: Icb3d284a2a952e2495d80fa91e22e0b32a54340f