'google-cloud-firestore 2.1.2' does not name 'packaging' as a dependency

See [this report](https://github.com/googleapis/python-firestore/pull/361#issuecomment-861716252):

```
  File "....python3.6/site-packages/google/cloud/firestore_v1/services/firestore/transports/base.py", line 18, in <module>
    import packaging.version
ModuleNotFoundError: No module named 'packaging'
```