When proposing #147 I noticed that mypy failed with missing
`types-request` - and while the mypy configuration had
the `--install-types`, it did not work automatically for requests.

The error was:

```
cherry_picker/cherry_picker.py:15: error: Library stubs not installed for
"requests"  [import-untyped]
    import requests
    ^
cherry_picker/cherry_picker.py:15: note: Hint: "python3 -m pip install types-requests"
cherry_picker/cherry_picker.py:15: note: (or run "mypy --install-types" to install all missing stub packages)
cherry_picker/cherry_picker.py:15: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
```

I looked a bit closer and noticed the advice from
https://github.com/pre-commit/mirrors-mypy

> Note that using the --install-types is problematic. Mutating the
pre-commit environment at runtime breaks cache and will break parallel
builds.

This PR fixes the problem by removing `--install-types` and installing
types-requests explicitly as additional-dependencies.

starting