Adjust for pint 0.20

[khaeru]

CI workflows for message_data (private) fail with messages like the following. Adjust.

Run pytest message_data \
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.8/x64/bin/pytest", line 8, in <module>
    sys.exit(console_main())
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 190, in console_main
    code = main()
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 148, in main
    config = _prepareconfig(args, plugins)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 329, in _prepareconfig
    config = pluginmanager.hook.pytest_cmdline_parse(
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_hooks.py", line 265, in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_manager.py", line 80, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 55, in _multicall
    gen.send(outcome)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/helpconfig.py", line 103, in pytest_cmdline_parse
    config: Config = outcome.get_result()
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_result.py", line 60, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 39, in _multicall
    res = hook_impl.function(*args)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1058, in pytest_cmdline_parse
    self.parse(args)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1346, in parse
    self._preparse(args, addopts=addopts)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1248, in _preparse
    self.hook.pytest_load_initial_conftests(
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_hooks.py", line 265, in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_manager.py", line 80, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 60, in _multicall
    return outcome.get_result()
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_result.py", line 60, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 39, in _multicall
    res = hook_impl.function(*args)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1125, in pytest_load_initial_conftests
    self.pluginmanager._set_initial_conftests(
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 557, in _set_initial_conftests
    self._try_load_conftest(anchor, namespace.importmode, rootpath)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 574, in _try_load_conftest
    self._getconftestmodules(anchor, importmode, rootpath)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 603, in _getconftestmodules
    mod = self._importconftest(conftestpath, importmode, rootpath)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 651, in _importconftest
    self.consider_conftest(mod)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 732, in consider_conftest
    self.register(conftestmodule, name=conftestmodule.__file__)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 496, in register
    self.consider_module(plugin)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 740, in consider_module
    self._import_plugin_specs(getattr(mod, "pytest_plugins", []))
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 747, in _import_plugin_specs
    self.import_plugin(import_spec)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 774, in import_plugin
    __import__(importspec)
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/message_ix_models/__init__.py", line 4, in <module>
    from iam_units import registry
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/iam_units/__init__.py", line 18, in <module>
    registry.load_definitions(str(Path(__file__).parent / "data" / "definitions.txt"))
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pint/facets/plain/registry.py", line 521, in load_definitions
    for definition in self._def_parser.iter_parsed_project(parsed_project):
  File "/opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/pint/delegates/txt_defparser/defparser.py", line 84, in iter_parsed_project
    raise stmt
pint.delegates.txt_defparser.common.DefinitionSyntaxError: Cannot define 'vkm' (AliasDefinition): the alias v km is not a valid unit alias (must not contain spaces)
    73,0-73,23 @alias vkm = vkt = v km
    /opt/hostedtoolcache/Python/3.10.8/x64/lib/python3.10/site-packages/iam_units/data/definitions.txt