pw_build: Support building Python wheels

Implement the .wheel subtarget for pw_python_package. This builds a
wheel for that Python package. Python wheels can be collected in a
directory using pw_mirror_tree's path_data_keys option for the "wheels"
key.

Fixed: 239
Change-Id: I43d756ce9714ba834b4590de702efeafc666b9ee
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/36762
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index c9399fb..7c4be4a 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -132,6 +132,7 @@
 # Requirements for the pw_python_package lint targets.
 pw_python_requirements("python_lint") {
   requirements = [
+    "build",
     "mypy==0.800",
     "pylint==2.6.0",
   ]