Flag --incompatible_load_python_rules_from_bzl will break rules_webtesting in Bazel 1.2.1

[bazel-flag-bot]

Incompatible flag --incompatible_load_python_rules_from_bzl will break rules_webtesting once Bazel 1.2.1 is released.

Please see the following CI builds for more information:

• [:ubuntu: 18.04 (OpenJDK 11)](:ubuntu: 18.04 (OpenJDK 11))
• [:windows: (OpenJDK 8)](:windows: (OpenJDK 8))
• [:darwin: (OpenJDK 8)](:darwin: (OpenJDK 8))

Questions? Please file an issue in https://github.com/bazelbuild/continuous-integration

Important: Please do NOT modify the issue title since that might break our tools.