```
scripts-dev/release.py:495: error: Unsupported right operand type for in ("Callable[[], IterableList[Reference]]")  [operator]
scripts-dev/release.py:496: error: Value of type "Callable[[], IterableList[Reference]]" is not indexable  [index]
```

`refs` is an alias the the property `references`. Mypy gets confused by
the alias, see python/mypy#6700

Mypy can't see that `tracking_branch()` returning truthy means that the
second call to `tracking_branch` returns a non-`None` value.

A use for the walrus operator?

`.reference` is a property whose setter function is `set_reference`.
My isn't happy when we try to assign directly:

```
scripts-dev/release.py:208: error: Trying to assign name "reference" that is not in "__slots__" of type "git.refs.head.HEAD"  [misc]
scripts-dev/release.py:208: error: Incompatible types in assignment (expression has type "HEAD", variable has type "Union[Head, TagReference, RemoteReference, Reference]")  [assignment]
```

but calling `set_reference` ourselves makes the typechecker happy.

Confusingly the following three types are all different:

- the `refererence` property: `Union['Head', 'TagReference', 'RemoteReference', 'Reference']`
- return type of `_get_reference`: `SymbolicReference`
- `set_reference` first argument: `Union[Commit_ish, 'SymbolicReference', str]`
  where `Commit_ish = Union['Commit', 'TagObject', 'Blob', 'Tree']`

It seems that typecheckers haven't settled down on The Way to handle
properties whose getter and setter types differ: see e.g.
python/mypy#3004 .

…-dev

This includes the changes to the release script to make it work
post-poetry.