tests: reduced memory footprint on subprocession tests. - Before those tests used ~4gb of RAM because of huge strings. Replacing it with generator buffers that should reduce memory usage.

dependencies: bumped gevent to 1.4.0

dependencies: bumped simplejson to 3.16.0

Backed out changeset f70ef45b551c. - evolve 8.3.3 turned out to be very unstable - https://bz.mercurial-scm.org/show_bug.cgi?id=6057

release: merge back stable branch into default

Added tag v4.15.2 for changeset a4dc36693455

release: Finish preparation for 4.15.2

release: updated pip2nix output for 4.15.2

release: Start preparation for 4.15.2

imports: fixed usage of exceptions in import statements as it's reserved in python3