CI: latest `ty` release breaks CI

[MarcoGorelli]

with the latest ty release, on the main branch, I see:

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:507:49
    |
506 | # DtypeArg specifies all allowable dtypes in a functions its dtype argument
507 | DtypeArg: TypeAlias = Dtype | Mapping[Hashable, Dtype]
    |                                                 ^^^^^
508 | DtypeObj: TypeAlias = np.dtype[np.generic] | ExtensionDtype
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:527:56
    |
526 | # converters
527 | ConvertersArg: TypeAlias = Mapping[Hashable, Callable[[Dtype], Dtype]]
    |                                                        ^^^^^
528 |
529 | # parse_dates
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:527:64
    |
526 | # converters
527 | ConvertersArg: TypeAlias = Mapping[Hashable, Callable[[Dtype], Dtype]]
    |                                                                ^^^^^
528 |
529 | # parse_dates
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:927:26
    |
925 |     | datetime.timedelta  # includes pd.Timedelta
926 | )
927 | S1 = TypeVar("S1", bound=SeriesDType, default=Any)
    |                          ^^^^^^^^^^^
928 | # Like S1, but without `default=Any`.
929 | S2 = TypeVar("S2", bound=SeriesDType)
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:929:26
    |
927 | S1 = TypeVar("S1", bound=SeriesDType, default=Any)
928 | # Like S1, but without `default=Any`.
929 | S2 = TypeVar("S2", bound=SeriesDType)
    |                          ^^^^^^^^^^^
930 | S2_contra = TypeVar("S2_contra", bound=SeriesDType, contravariant=True)
931 | S2_NDT_contra = TypeVar(
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:930:40
    |
928 | # Like S1, but without `default=Any`.
929 | S2 = TypeVar("S2", bound=SeriesDType)
930 | S2_contra = TypeVar("S2_contra", bound=SeriesDType, contravariant=True)
    |                                        ^^^^^^^^^^^
931 | S2_NDT_contra = TypeVar(
932 |     "S2_NDT_contra", bound=SeriesDTypeNoDateTime, contravariant=True
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:932:28
    |
930 | S2_contra = TypeVar("S2_contra", bound=SeriesDType, contravariant=True)
931 | S2_NDT_contra = TypeVar(
932 |     "S2_NDT_contra", bound=SeriesDTypeNoDateTime, contravariant=True
    |                            ^^^^^^^^^^^^^^^^^^^^^
933 | )
934 | S2_NSDT = TypeVar("S2_NSDT", bound=SeriesDTypeNoStrDateTime)
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:934:36
    |
932 |     "S2_NDT_contra", bound=SeriesDTypeNoDateTime, contravariant=True
933 | )
934 | S2_NSDT = TypeVar("S2_NSDT", bound=SeriesDTypeNoStrDateTime)
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^
935 | S3 = TypeVar("S3", bound=SeriesDType)
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:935:26
    |
933 | )
934 | S2_NSDT = TypeVar("S2_NSDT", bound=SeriesDTypeNoStrDateTime)
935 | S3 = TypeVar("S3", bound=SeriesDType)
    |                          ^^^^^^^^^^^
936 |
937 | # Constraint, instead of bound
    |
info: rule `invalid-type-form` is enabled by default

error[invalid-type-form]: Variable of type `UnionType` is not allowed in a type expression
   --> pandas-stubs/_typing.pyi:948:5
    |
946 |     float,
947 |     complex,
948 |     Dtype,
    |     ^^^^^
949 |     datetime.datetime,  # includes pd.Timestamp
950 |     datetime.timedelta,  # includes pd.Timedelta
    |
info: rule `invalid-type-form` is enabled by default

With the previous release, everything passes. I think that's what's affecting the failures in #1481

[Dr-Irv]

See my comment here: #1376 (comment)

[loicdiridollou]

Created astral-sh/ty#1534