contrib/hgweb.wsgi

pep-517: remove the `build-backend` key to allow for users to build extensions

PEP 517 changed how projects should define their build dependencies.
The presence of a `pyproject.toml` file changes the behavior of `pip` to
conform to PEP 517.
Since we haven't updated the `setup.py` file yet (I'm not even sure we're not
an edge case which would make it harder/impossible, I have not tried yet), this
is a workaround to issue6589: it allows users to pass `--no-use-pep517` to pip
to revert to the legacy installer. The build backend is simply assumed to be
the `build_meta:__legacy__` one.

Differential Revision: https://phab.mercurial-scm.org/D11393

# An example WSGI for use with mod_wsgi, edit as necessary
# See https://mercurial-scm.org/wiki/modwsgi for more information

# Path to repo or hgweb config to serve (see 'hg help hgweb')
config = b"/path/to/repo/or/config"

# Uncomment and adjust if Mercurial is not installed system-wide
# (consult "installed modules" path from 'hg debuginstall'):
#import sys; sys.path.insert(0, "/path/to/python/lib")

# Uncomment to send python tracebacks to the browser if an error occurs:
#import cgitb; cgitb.enable()

# enable demandloading to reduce startup time
from mercurial import demandimport; demandimport.enable()

from mercurial.hgweb import hgweb
application = hgweb(config)