Dependabot does not like constraints in the requirements.*.in files

[StevenMaude]

Despite this being a common pip pattern, Dependabot does not like them.

Really this is a Dependabot issue, but has an impact on repositories created from this template, because those new repositories then have this problem.

Hopefully Dependabot gets a fix, but it's been an open issue since 2020, so who knows when.

The alternative is that we encourage use of pyproject.toml to specify dependencies.

[StevenMaude]

uv might also help here just as in #52, maybe? See #263.

[StevenMaude]

We're no longer using Dependabot for Python package updates; closing.