v3 no longer accepts fields that call class methods

Nobody has claimed this yet.

While upgrading from v2.21.0 to v3.10.0, I have found a difference in behaviour that I could not find in the Changelog / update docs.

I have created a contrived case to show the issue:
```
import marshmallow

class Test(marshmallow.Schema):
number = marshmallow.fields.Number()

class TestObject(object):
def number(self):
return 1

if marshmallow.__version__.startswith("3"):
print(Test().dump(TestObject()))
else:
print(Test().dump(TestObject()).data)
```

When run in `marshmallow==2.21.0`, the output is:
```
{'number': 1.0}
```

while when run in `marshmallow==3.10.0`, I get this error:
```
TypeError: float() argument must be a string or a number, not 'method'
```

Can anybody shed some light on this difference in behaviour? Is it to be expected or is it a bug?

If it is expected, what is the best way to deal with this for the upgrade?

Start by running the supplied marshmallow Schema and TestObject reproduction against versions 2.21.0 and 3.10.0, then trace the dump behavior for the field whose value is a class method. Done means the version difference is explained and the issue has either documented upgrade guidance or a confirmed regression with a focused regression test.

Written by the indexing model from the issue text.