pyproject.toml: Add uv as an "extra" dependency for FawltyDeps

This allows users to install (or depend on) `"fawltydeps[uv]"` which
will then automatically bring in `uv` alongside `fawltydeps`.

This has no effect on our test/developer environments, as we install
`fawltydeps` without extras, but we still get `uv` via our dependency
on `nox[uv]`.

We should consider updating the FawltyDeps GitHub Action to install
`fawltydeps[uv]`, so that we can benefit from `uv` in that scenario as
well.

README.md

@@ -248,7 +248,8 @@ section of your `pyproject.toml`.
 
 FawltyDeps will use [`uv`](https://github.com/astral-sh/uv) by default to
 temporarily install missing dependencies. If `uv` not available, `pip` will be
-used instead.
+used instead. If you want to ensure that the faster `uv` is avaiable, you can
+install `fawltydeps` with the `uv` extra (e.g. `pip install fawltydeps[uv]`).
 
 To further customize how this automatic installation is done (e.g. if you need
 to use a different package index), you can use environment variables to alter

pyproject.toml

@@ -55,6 +55,12 @@ setuptools = [
     {version = ">=68.0.0,<68.1.0", python = "<3.8"},
 ]
 
+# Optional dependencies, covered by .extras below:
+uv = {version = ">=0.1.6", python = ">=3.8", optional = true}
+
+[tool.poetry.extras]
+uv = ["uv"]
+
 [tool.poetry.group.nox]
 optional = true