Release v0.3.1

Features

• The Linux AppImage backend has been modified to use Docker. This ensures that
  the AppImage is always built in an environment that is compatible with the
  support package. It also enables Linux AppImages to be built on macOS and
  Windows. "Native" builds (i.e., builds that don't use Docker) can be invoked
  using the --no-docker argument. (#344)
• A PYTHONPATH property has been added to AppConfig that describes the
  sys.path changes needed to run the app. (#401)
• Ad-hoc code signing is now possible on macOS with briefcase package --adhoc-sign. (#409)
• Android apps can now use use - in their bundle name; we now convert -
  to _ in the resulting Android package identifier and Java package name. (#415)
• Mobile applications now support setting the background color of the splash
  screen, and setting a build identifier. (#422)
• Android now has a package command that produces the release APK. After
  manually signing this APK, it can be published to the Google Play Store. (#423)

Bugfixes

• Some stray punctuation in the Android device helper output has been removed. (#396)
• An explicit version check for Docker is now performed. (#402)
• The Linux build process ensures the Docker user matches the UID/GID of the host
  user. (#403)
• Briefcase now ensures that the Python installation ecosystem tools (pip,
  setuptools, and wheel), are all present and up to date. (#421)

Improved Documentation

• Documented that Windows MSI builds produce per-user installable MSI installers,
  while still supporting per-maching installs via the CLI. (#382)
• CONTRIBUTING.md has been updated to link to Briefcase-specific
  documentation. (#404)
• Removed references to the build-system table in pyproject.toml. (#410)