Pin virtualenv to 16.7.9

[lidizheng]

Fixes #21966

In short, virtualenv >= 20.0.0 + realpath doesn't work.

virtualenv introduces a breaking change in their 20.0.x release, where the Python binary changed from a script to a symlink. Our infrastructure code depended on previous behavior, and invokes realpath on the Python binary to get the full file path. However, for symlinks, realpath always traces down the symlinks and eventually gets the system version of Python. Then, the build script installed gRPC Python packages in the wrong Python environment, resulted in test failure.

root@4a3f61c8c7a9:/var/local/git/grpc# ls -l py37_asyncio/bin/python
lrwxrwxrwx 3 root staff 18 Feb 11 18:45 py37_asyncio/bin/python -> /usr/bin/python3.7
root@4a3f61c8c7a9:/var/local/git/grpc# realpath py37_asyncio/bin/python
/usr/bin/python3.7

---

Manual sanity check passed:

tools/run_tests/run_interop_tests.py -l python -s python --use_docker --http2_interop --internal_ci -t -j 1

[lidizheng]

Known failures: #13379