[python3] add __pycache__ to .gitignore.
[python3] add __pycache__ to .gitignore.

R=yuanjunh

No-Try: true
Change-Id: I96d57508b6c0b174eb1add12b21e539f7864beb8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3202591
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Reviewed-by: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>

luci/recipes-py,luci/recipes-py