Regression: SchemaModel.field no longer resolves to `str` (descriptor typing needed)

[cristianmatache]

Code Sample, a copy-pastable example

import pandera.typing as pat
from pandera import SchemaModel

class Sch(SchemaModel):
    col: pat.Series[int]

print(type(Sch.col))   # str
reveal_type(Sch.col)   # note: Revealed type is "pandera.typing.pandas.Series[builtins.int]"

import pandera.typing.koalas as patk

class Sch2(SchemaModel):
    col: patk.Series[int]

print(type(Sch2.col))   # str
reveal_type(Sch2.col)   # note: Revealed type is "pandera.typing.koalas.Series[builtins.int]"

Expected behavior

The expected type should be str.

[cosmicBboy]

fixed by #739