Here's a patch to soften the impact of pyc-related problems.

Note:
* Each pyc-related case that previously caused import to fail now results in an ImportWarning.  This applies to both reading and writing .pyc files.
* Each warning has __cause__ set to the caught exception.
* Some cases where the exception was already being swallowed now result in a warning while others are still swallowed.
* The tests verify that the warnings, with appropriate causes, are issued.
* The tests verify that existing bad .pyc file get overwritten.
* importlib._bootstrap_external.StaleBytecodeError is new.  It should surface only from SourcelessFileLoader.
* SourcelessFileLoader is not changed.