Use all upper-case for Python constants. Also use line breaks to make
the templates more readable.

No functional changes, refactoring only.

Bug: angleproject:2621
Change-Id: I3ec1c07089b33fd1655ffbacc8c3a8cdcb919a3b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2553967
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>