Use proxies for @injectable in the py-envs component.

[ericsnowcurrently]

In #12795 we created proxies for all the "injected" classes in the py-envs component to eliminate internal use of @injectable. However, this caused problems (see #12833). So we reverted in #13278.

We need to un-revert, but do it one piece at a time so we can address the source of the problem described in #12833.

[ericsnowcurrently]

FWIW, the revert may have been unnecessary. (The same tests are still flaky and it certainly seemed unlikely that #12795 was the source of that problem.) So a straight un-revert may be all that's needed.