Oh, the "Run mypy on Lib/tomllib" job fails with "Name frozendict is not defined" because it runs on an older Python version which doesn't have this new built-in type:

Run mypy --config-file Lib/tomllib/mypy.ini
Lib/tomllib/_parser.py:43: error: Name "frozendict" is not defined 
[name-defined]
    BASIC_STR_ESCAPE_REPLACEMENTS: Final = frozendict(
                                           ^~~~~~~~~~
Found 1 error in 1 file (checked 4 source files)
Error: Process completed with exit code 1.

I reverted the tomllib change to workaround the mypy issue.

cc @picnixz

Oh, tests are failing

0:13:17 load avg: 10.90 [1/1/1] test_dataclasses failed (1 failure)
Re-running test_dataclasses in verbose mode (matching: test_field_repr)
test_field_repr (test.test_dataclasses.TestCase.test_field_repr) ... FAIL

FAIL: test_field_repr (test.test_dataclasses.TestCase.test_field_repr)

Traceback (most recent call last):
File "/Users/runner/work/cpython/cpython/Lib/test/test_dataclasses/init.py", line 79, in test_field_repr
self.assertEqual(repr_output, expected_output)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: "Fiel[159 chars]dict(),kw_only=<dataclasses._MISSING_TYPE obje[50 chars]one)" != "Fiel[159 chars]dict({}),kw_only=<dataclasses._MISSING_TYPE ob[52 chars]one)"

I'll have a look on this as soon as I've got some time

I updated my branch and I updated test_dataclasses for repr(frozendict) change.

metadata=frozendict({})

I updated repr(frozendict) of empty frozendict to: frozendict() instead of frozendict({}).

LGTM

Merged. Thanks for reviews.