tests: fix http-bad-server expected errors for python 3.10 (issue6643)
The format of the error message changed with this version of Python.
This also removes obsolete Python 3 checks.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls -A
.hg
a
unknown