Encode Python indentation style in .editorconfig

Infuriatingly, `git cl format` and our PRESUBMIT scripts enforce
4-space indentation for Python scripts, even though we use 2-space
indentation everywhere else. This patch encodes that preference at
the .editorconfig level.

https://google.github.io/styleguide/pyguide.html#34-indentation

Change-Id: Ibe31afe6cd48fe762e04578687802da61b4b935f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2248197
Auto-Submit: Mathias Bynens <[email protected]>
Commit-Queue: Paul Lewis <[email protected]>
Reviewed-by: Paul Lewis <[email protected]>

Author: mathiasbynens

.editorconfig

@@ -7,3 +7,6 @@ end_of_line = lf
 charset = utf-8
 trim_trailing_whitespace = true
 insert_final_newline = true
+
+[*.py]
+indent_size = 4