Don't report coverage when using the --py3-only flag.
Don't report coverage when using the --py3-only flag.

When using the --py3-only flag without any filters, the coverage report
gets displayed at the end of the test run. But since any modules or
recipes that haven't been updated to declare compatibility with python3
will not end up getting covered, this can result in large amounts of
spurious output. Preventing the output when the --py3-only flag is set
will reduce the amount of noise while iterating for the migration.

Change-Id: Idbe63218264cc5d80375268b94d32fce9b7bb798
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3301059
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Garrett Beaty <a0767768af30bcbe01b6de5024355c539362a830@google.com>

luci/recipes-py,luci/recipes-py