Require at least setuptools version 61

Use of the automatic discovery feature was intoduced with PR matrix-org#27. The
feature is only available in setuptools version 61 and later, though,
which this change documents in pyproject.toml.

Author: V02460

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools", "wheel"]
+requires = ["setuptools>=61", "wheel"]
 build-backend = "setuptools.build_meta"
 
 [tool.isort]