This allows findsource() to work in doctests.

A patch from Dirkjan Ochtman.

methods.

Original patch by Erik Bray.

Original patch by Erik Bray.

Original patch by Erik Bray as part of #11595, changed by me to improve
readability.

Original patch by Tshepang Lekhonkhobe.

Just try to open files from the ZIP for reading, don't extract them to avoid
UnicodeEncodeError if the filename is not encodable to the filesystem encoding
(e.g. ASCII locale encoding).

The new tests now requires pthread_sigmask(). Skip the test if the function is
missing, e.g. if Python is compiled without threads.