util: eliminate the possibility of returning None from `versiontuple()`
This fixes the following error flagged by pytype:
File "/mnt/c/Users/Matt/hg/mercurial/extensions.py", line 228, in load: unsupported operand types for > [unsupported-operands]
Primitive types 'Optional[tuple]' and 'curver: Optional[tuple]' are not comparable.
Differential Revision: https://phab.mercurial-scm.org/D11473
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL