[3.6] bpo-31234: Join threads in tests (#3589)

[3.6] bpo-31234: Join threads in tests (#3589)

* bpo-31234: Join threads in tests (#3572)

Call thread.join() on threads to prevent the "dangling threads"
warning.

(cherry picked from commit 18e95b4176256f100429a806d0455406df98f984)

* bpo-31234: Join threads in test_hashlib (#3573)

* bpo-31234: Join threads in test_hashlib

Use thread.join() to wait until the parallel hash tasks complete
rather than using events. Calling thread.join() prevent "dangling
thread" warnings.

* test_hashlib: minor PEP 8 coding style fixes

(cherry picked from commit 8dcf22f442320e4c1a5408e67b4c9002ad105f17)

* bpo-31234: Join threads in test_threading (#3579)

Call thread.join() to prevent the "dangling thread" warning.

(cherry picked from commit b8c7be2c523b012e57915182543d06657161057f)

* bpo-31234: Join threads in test_queue (#3586)

Call thread.join() to prevent the "dangling thread" warning.

(cherry picked from commit 167cbde50a88ec2a7d26b2cb9891d5e32bdfbfb5)

* bpo-31234: Join timers in test_threading (#3598)

Call the .join() method of threading.Timer timers to prevent the
"threading_cleanup() failed to cleanup 1 threads" warning.

(cherry picked from commit da3e5cf961f9bcc4bb376386cfe7a2865325086c)