Adopt Python example

Python tree with an app package, a stdlib-only solo directory, and a tests/ directory.

bazel run @rules_dx//:dx -- init //examples/adopt-python/...
bazel run @rules_dx//:dx -- generate //examples/adopt-python/...
bazel build //examples/adopt-python/...
bazel test //examples/adopt-python/...

All 4 tests pass (handlers_test, pure_test, pure_unittest_test, login_test).