.gitignore: Ignore local files written by pyenv

Change-Id: I225c12064ba06b6f5e9c5fc5dad3249830e06942
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/193001
Reviewed-by: Austin Eng <enga@chromium.org>
Auto-Submit: David Neto <dneto@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

.gitignore

diff --git a/.gitignore b/.gitignore
index 0fb5648..14ddf65 100644
--- a/.gitignore
+++ b/.gitignore
@@ -153,3 +153,6 @@
 all_findings.json
 
 tint.dot
+
+### pyenv files
+/.python-version