Add transform for Python requirements installation

[rogerinhoam]

Summary

• add a python-requirements taskgraph transform for one or more requirements files
• migrate evaluate-teacher-ensemble away from an inline requirements install
• validate the generated command and error when the expected virtualenv activation is missing

Testing

• python -m unittest taskcluster.test.test_python_requirements -v
• taskgraph full --json --parameters taskcluster/test/params/small-ru-en.yml (153 tasks, 457 dependencies)
• compared the generated graph against main; both files have the same SHA-256 hash

Addresses #588.

[evgenyrp]

LGTM if it works. To test it in CI, we'll need to land first because it's an untrusted fork.

[bhearsum]

LGTM if it works. To test it in CI, we'll need to land first because it's an untrusted fork.

For future reference, you can mark any tasks that don't require scopes for things that shouldn't be available to non-collaborators as being able to run on untrusted pull requests. (See taskcluster documentation and an example from another repo.)

[evgenyrp]

LGTM if it works. To test it in CI, we'll need to land first because it's an untrusted fork.

For future reference, you can mark any tasks that don't require scopes for things that shouldn't be available to non-collaborators as being able to run on untrusted pull requests. (See taskcluster documentation and an example from another repo.)

@bhearsum I added it for tests and linter: #1448

[evgenyrp]

Linting failed; please run task lint-fix. See https://mozilla.github.io/translations/docs/contributing/development/