pw_build: pw_python_package template

- The pw_python_package template brings Python code into the GN build
  and pw watch. Supports mypy, pylint, and test running. Package
  installation and wheel building will be supported in the future.
- Declare the pw_tokenizer package with pw_python_package as an example.
- Provide a top-level "python" GN target and add Python linting and test
  running to the default group.

Bug: 239
Change-Id: Iebc47dc4e23d911600ca0911d831910f69e66aca
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/20641
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/.pylintrc b/.pylintrc
index 53635ca..3b3216d 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -61,7 +61,8 @@
 # --enable=similarities". If you want to run only the classes checker, but have
 # no Warning level messages displayed, use "--disable=all --enable=classes
 # --disable=W".
-disable=fixme,
+disable=bad-continuation,  # Rely on yapf for formatting
+        fixme,
         subprocess-run-check
 
 # Enable the message, report, category or checker with the given id(s). You can