We build the .pip files on Mac.  Some transitive dependencies are
different on other OS's, and so would fail there, like this:

```
ERROR: In --require-hashes mode, all requirements must have their versions pinned with ==. These do not:
    SecretStorage>=3.2 from https://files.pythonhosted.org/packages/54/24/b4293291fa1dd830f353d2cb163295742fa87f179fcc8a20a306a81978b7/SecretStorage-3.3.3-py3-none-any.whl#sha256=f356e6628222568e3af06f2eba8df495efa13b3b63081dafd4f7d9a7b7bc9f99 (from keyring==23.13.1->-r requirements/dev.pip (line 279))
```

By keeping the .in files self-contained, we provide a way to install
without hashes.