Introduce lower bound on attrs version (#179)

`lsprotocol` uses the `import attrs` API which was only introduced in
[`v21.3.0`](https://www.attrs.org/en/stable/changelog.html#id7) .

This PR sets a minimum version for attrs in the python package's
`pyproject.toml` to hopefully guard against issues [like this
one](swyddfa/esbonio#147 (comment))
in the future.

**Question:** Is it sufficient to rely on the version bound set here to
ensure the correct version is also installed for `pygls` and `esbonio`?
Or since both downstream packages also `import attrs` themselves, is it
more correct to have a version bound set in both `pygls` and `esbonio`?
:thinking:

packages/python/pyproject.toml

@@ -27,7 +27,7 @@ classifiers = [
   "Programming Language :: Python :: Implementation :: CPython",
   "Programming Language :: Python :: Implementation :: PyPy",
 ]
-dependencies = ["attrs", "cattrs"]
+dependencies = ["attrs>=21.3.0", "cattrs"]
 
 [project.urls]
 Issues = "https://github.com/microsoft/lsprotocol/issues"