hgweb: simplify uenv assignment
We don't need the Python 3 conditional. We can call items() directly
since we're on Python 3 now.
Differential Revision: https://phab.mercurial-scm.org/D12307
chgserver: remove Python 2 support code
The logic here is more complicated than most Python 2/3 support code.
But the rewritten logic should be identical.
Differential Revision: https://phab.mercurial-scm.org/D12306