merge with stable

makefile: use Python 3 by default (BC) This change is long overdue IMO. .. bc:: Makefile now uses `python3` instead of `python` by default on non-Windows platforms. This means Mercurial will be built and run with Python 3 instead of Python 2.7 by default. To continue using Python 2, set the PYTHON variable. e.g. `make install PYTHON=python2.7`. Differential Revision: https://phab.mercurial-scm.org/D7258

hgweb: don't call sys.exit() in httpservice.run() If I'm reading the code correctly, `mercurial.server.createservice()` can return an hgweb service or one of three types of command server services. The caller then calls `mercurial.server.runservice()`, passing it the returned service's run method. Only the hgweb service was calling `sys.exit()`. It has been that way since 8d44649df03b (refactor ssh server., 2006-06-04). That commit message doesn't provide any explanation. Let's clean up and have the code follow the usual return path into the `dispatch` module. After this patch, there should be no remaining places left where we call `sys.exit()` except for valid uses in the `dispatch` and `worker` modules. Differential Revision: https://phab.mercurial-scm.org/D9272