Description

GeometryField (MySQL) subclasses Field directly with a single base:

class GeometryField(Field):
    SQL_TYPE = "GEOMETRY"

The _FieldMeta metaclass only auto-assigns field_type when a field has
multiple bases (len(bases) > 1 and bases[0] is Field). A single-base
subclass therefore keeps the class default field_type = None.

The base value converters use it directly:

def to_db_value(self, value, instance):
    ...
    if isinstance(value, self.field_type):   # isinstance(value, None)

So any non-None value round-tripping through GeometryField raises:

TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

This trips both to_python_value (reading a geometry back from the DB) and
to_db_value (writing one).

Fix

Declare a concrete field_type and parametrize the generic, exactly as
#2237 did for
TSVectorField:

class GeometryField(Field[str]):
    SQL_TYPE = "GEOMETRY"
    field_type = str

Tests

Added tests/contrib/mysql/test_geometry_field.py (DB-free) asserting
field_type is str and that to_python_value/to_db_value handle both
None and a concrete "POINT(1 1)" value without raising. Reproduces as a
TypeError before the fix; passes after.

tests/fields/, tests/schema/test_generate_schema.py and the new test all
pass (474 passed, 66 skipped); ruff format --check and ruff check clean.