Add webpack commands into setup.py

This is a POC that shows building webpack through standard `setup.py`
commands. Any call to `setup.py build` or `bdist` or `sdist` will
trigger a Webpack build of the assets first. A non-zero exit code will
halt the process.

Also, moved the `npm run dev` command, which here is `python setup.py
watch`, though there is perhaps something better here. There is already
`python setup.py develop`, which has a separate function, so I don't
want to collide there.

Example output here:
https://gist.github.com/agjohnson/cdaab364fe598daa7f3bef750cfb84dd

Refs #797

docs/contributing.rst

@@ -94,9 +94,11 @@ To release a new version of the theme, core team will take the following steps:
    (with regards to alpha release and development versions). The version
    increment should reflect these releases and any potentially breaking changes.
 #. Update the changelog (``docs/changelog.rst``) with the version information.
-#. Run ``npm run build`` to rebuild all the theme assets.
-#. Run ``python setup.py update_translations`` to compile new translation files and update Transifex
-#. Commit that change.
+#. Run ``python setup.py update_translations`` to compile new translation files
+   and update Transifex.
+#. Run ``python setup.py build`` to rebuild all the theme assets and the Python
+   package.
+#. Commit these changes.
 #. Tag the release in git: ``git tag $NEW_VERSION``.
 #. Push the tag to GitHub: ``git push --tags origin``.
 #. Upload the package to PyPI:

setup.py

@@ -7,10 +7,39 @@
 
 import subprocess
 import distutils.cmd
+import setuptools.command.build_py
 from io import open
 from setuptools import setup
 
 
+class WebpackBuildCommand(setuptools.command.build_py.build_py):
+
+    """Prefix Python build with Webpack asset build"""
+
+    def run(self):
+        subprocess.run(['webpack', '--config', 'webpack.prod.js'], check=True)
+        super().run()
+
+
+class WebpackDevelopCommand(distutils.cmd.Command):
+
+    description = "Run Webpack dev server"
+
+    user_options = []
+
+    def initialize_options(self):
+        pass
+
+    def finalize_options(self):
+        pass
+
+    def run(self):
+        subprocess.run(
+            ["webpack-dev-server", "--open", "--config", "webpack.dev.js"],
+            check=True
+        )
+
+
 class UpdateTranslationsCommand(distutils.cmd.Command):
 
     description = "Run all localization commands"
@@ -47,8 +76,8 @@ class TransifexCommand(distutils.cmd.Command):
         pass
 
     def run(self):
-        subprocess.run(['tx', 'push', '--source'])
-        subprocess.run(['tx', 'pull'])
+        subprocess.run(['tx', 'push', '--source'], check=True)
+        subprocess.run(['tx', 'pull'], check=True)
 
 
 setup(
@@ -63,6 +92,8 @@ setup(
     cmdclass={
         'update_translations': UpdateTranslationsCommand,
         'transifex': TransifexCommand,
+        'build_py': WebpackBuildCommand,
+        'watch': WebpackDevelopCommand,
     },
     zip_safe=False,
     packages=['sphinx_rtd_theme'],