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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a