fsmonitor: migrate Python ABCs from collections to collections.abc The Collections Abstract Base Classes in the collections module are deprecated since Python 3.3 in favor of collections.abc, and removed in Python 3.10.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL