Over in packaging, that code expects `sysconfig.get_config_var("py_version_nodot")` to be consistent with `tags._version_nodot`, which expects 3_10 for python 3.10. See https://github.com/pypa/packaging/issues/308. The current value of `sysconfig.get_config_var("py_version_nodot")` (which comes from https://github.com/python/cpython/blob/master/Lib/sysconfig.py#L89) is `310` which is ambiguous.

Links about the "AssertionError: would build wheel with unsupported tag ('cp310', 'cp310', 'linux_x86_64')" error:

* PEP 641 -- Using an underscore in the version portion of Python 3.10 compatibility tags
  https://www.python.org/dev/peps/pep-0641/
* PEP 641 discussion:
  https://discuss.python.org/t/pep-641-using-an-underscore-in-the-version-portion-of-python-3-10-compatibility-tags/5513
* CPython PR 20333: "bpo-40747: Make py_version_nodot 3_10 not 310"
  https://github.com/python/cpython/pull/20333
* wheel: "Fails to build wheel for Python 3.10"
  https://github.com/pypa/wheel/issues/354
* python3-setuptools: "python-setuptools fails to build with Python 3.10: AssertionError: would build wheel with unsupported tag ('cp310', 'cp310', 'linux_x86_64')"
https://bugzilla.redhat.com/show_bug.cgi?id=1891840

What's blocking this?
It is a real pain not to be able to install packages for 3.10.

I don't much care what the format is, but since there seems to be some difficulty in changing it, why not leave the output as `310`?

It isn't ambiguous, as long as we specify how to parse the number:
2 digits: XY.  X = major, Y = minor
3 digits: XYY. X = major, YY = minor
4 digits: XXYY. XX = major, YY = minor

So 31.0 will be `3100`.

That should be good for a millenium or so :)

This has been blocking it: https://discuss.python.org/t/pep-641-using-an-underscore-in-the-version-portion-of-python-3-10-compatibility-tags/5513

Pablo (PEP delegate) has just posted his rejection though, so it will be fixed when packaging changes back to "310" from "3_10" and updates percolate out through the ecosystem.

Since moving to "4.x" is a breaking change, we can change to any system we like at that point. Well in advance of 31.0 being ambiguous.