* tools/buildgen/extract_metadata_from_bazel_xml.py changes from #25272
* regenerate projects
* get rid of useless _deduplicate_append
* extract_deps worked for both collapsed_sources and collapsed_deps only due to mismatch of bazel label vs target name mismatch
* special handling for absl has no effect
* improve comments
* remove changes that are not needed yet
* Ran `2to3` on `tools/buildgen` and the `gen_build_yaml.py` files.
* Updated the invocations of python to be explicitly Python 3.
* Changed the loader in `mako_renderer.py` to allow pickle to find the module for pickling `external_version.Version`
* Added a few open-as-binary to placate readers/writers expecting bytes.
* Hand tweaked the templates to remove `iteritems` and fiddle with encodings.
Partially addresses #24359.