* 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.
Alexander Polcyn