regression on -current with mozjs78

mozjs78 doesn't build anymore on -current
------------------------------
|--- a/config/run_spidermonkey_checks.py
|+++ b/config/run_spidermonkey_checks.py
--------------------------
patching file config/run_spidermonkey_checks.py
Using Plan A...
Hunk #1 succeeded at 12 (offset 1 line).
done
Traceback (most recent call last):
File "/tmp/firefox-78.15.0/js/src/./../../configure.py", line 25, in <module>
from mozbuild.configure import (
File "/tmp/firefox-78.15.0/python/mozbuild/mozbuild/configure/__init__.py", line 33, in <module>
from mozbuild.util import (
File "/tmp/firefox-78.15.0/python/mozbuild/mozbuild/util.py", line 760, in <module>
class HierarchicalStringList(object):
File "/tmp/firefox-78.15.0/python/mozbuild/mozbuild/util.py", line 785, in HierarchicalStringList
class StringListAdaptor(collections.Sequence):
AttributeError: module 'collections' has no attribute 'Sequence'
make: *** No targets specified and no makefile found. Stop.
make: *** No targets specified and no makefile found. Stop.
---------------------------
No need to say who is to blame.