Add support for Python 3.15 - #206

Python 3.15 changed doctest.DocTestCase to report each example as a
separate subtest (python/cpython#108885).  The subtest reports happen
while doctest has sys.stdout redirected to its internal buffer, so
zope.testrunner's immediate failure output was swallowed by that buffer
and corrupted the output of the following examples.

Restore the pre-3.15 behavior of raising a single failure for the whole
doctest after sys.stdout has been restored.

Also allow for the new 'Did you mean' suggestion in ModuleNotFoundError
messages on Python 3.15 and revert the report file count workaround
from #205 which is no longer needed.