Docs: remove autodocs for removed modules

We forgot to remove the docs for:
translate/misc/{ini,context,contextlib}.py
Docs: include next release in doctree
Docs: report warnings as errors

This should allow us to catch autodoc related errors and files not in
the toctree.
Travis: move standalone pip installs to requirements/dev.txt
Travis: use pytest-xdist to test in subprocesses

This helps testing when using cPO, fPO and pypy that may segfault.
tools/buildxpi: Switch from optparse to argparse
get_moz_enUS: Switch to argparse
get_moz_enUS: Use python3-compatible print syntax
moz_l10n_builder: Switch to argparse
moz_l10n_builder: Use python3-compatible print syntax
mozcronbuild: Switch to argparse