PEP 780: Update to use sys.abi_info

peps/pep-0780.rst

@@ -284,14 +284,23 @@ Require Free-Threaded Fork of PyYAML
 ------------------------------------
 PyYAML initially decided not to support free-threading in its mainline
 distribution before wider testing of free-threaded Python in the real world.
-To faciliate this testing, a free-threaded fork, PyYAML-ft, was created.
+To facilitate this testing, a free-threaded fork, PyYAML-ft, was created.
 To require this fork, for a free-threaded Python interpreter, without forcing
 it also on the GIL-enabled interpreter, the following dependency specification
 can be used::
 
     pyyaml-ft; "free_threaded::True" in sys_abi_info
     pyyaml; "free_threaded::True" not in sys_abi_info
 
+Note that in the absence of the features described in this PEP, early adopting
+projects have had to resort to using the fork for *all* builds of a potentially
+free threading Python interpreter; typical dependencies than look like::
+
+    pyyaml; python_version<"3.13"
+    pyyaml-ft; python_version>="3.13"
+
+which is an undesirable side effect for GIL-enabled environments.
+
 Backwards Compatibility
 =======================
 
@@ -304,8 +313,8 @@ ecosystem tools, especially those which attempt to support examination of data
 in ``pyproject.toml`` and ``requirements.txt``.
 
 In particular, current tools generally work with a fixed list of environment
-variables, either in their own implementation (e.g. `uv <https://github.com/astral-sh/uv/blob/86e7b2e97a193ce3f506804be4c41c611191fa72/crates/uv-pep508/src/marker/tree.rs#L170>`_)
-or via an established package like `pypa/packaging <https://github.com/pypa/packaging/blob/8f13a4309aa620bf643cd508304f783b456e8c49/src/packaging/_tokenizer.py#L63>`_ (e.g. pip).
+variables, either in their own implementation (e.g. `uv`_) or via an
+established package like `pypa/packaging`_ (e.g. pip).
 Hence, it is principally impossible to add any new environment markers without
 making new packages using these markers incompatible with older versions of
 these tools.
@@ -415,6 +424,8 @@ Footnotes
    particularly on platforms that allow the execution of either kind of binary.
 
 
+.. _uv: https://github.com/astral-sh/uv/blob/86e7b2e97a193ce3f506804be4c41c611191fa72/crates/uv-pep508/src/marker/tree.rs#L170
+.. _pypa/packaging: https://github.com/pypa/packaging/blob/8f13a4309aa620bf643cd508304f783b456e8c49/src/packaging/_tokenizer.py#L63
 .. _Python experimental support for free threading: https://docs.python.org/3/howto/free-threading-python.html
 .. _Python free-threading guide: https://py-free-threading.github.io/
 .. _Environment marker for free-threading: https://discuss.python.org/t/environment-marker-for-free-threading/60007