hgweb: enable reading styles from resources in frozen binaries
All we need to do to read styles from resources is to pass the
file-like object we get from `open_template()` on to `frommapfile()`.
This takes the number of failing tests with PyOxidizer from 62 to 54.
Differential Revision: https://phab.mercurial-scm.org/D8901
running: init test1
result: 0
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: 0
running: commit -m commit2 -d 2000-01-02 foo
result: 0
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: 0
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: 0