Newer stestr is needed to avoid failures on Python 3.11 like:
nox > stestr run --color
stestr: 'run --color' is not a stestr command.
Go ahead and uncap all the test dependencies. Also drop the pbr
addition for Python<=3.5 since we no longer test with anything older
than 3.6 anyway, and fix an exception that hacking seems to really
not want us to use.
Change-Id: I85585324c4e47d37b6de555110066fb8c29f293f
d879220c10