Add <4 to require as recommended by Flit docs

As per https://flit.pypa.io/en/stable/pyproject_toml.html#build-system-section:

> For now, all packages should specify <4, so they won't be
> impacted by changes in the next major version.

Author: not-my-profile

source/shared/build-backend-tabs.rst

@@ -22,7 +22,7 @@
     .. code-block:: toml
 
         [build-system]
-        requires = ["flit_core >= 3.12.0"]
+        requires = ["flit_core >= 3.12.0, <4"]
         build-backend = "flit_core.buildapi"
 
 .. tab:: PDM