add support for Any annotation in schema model

the motivation behind this feature is to support column annotations
that can have any type, to support use cases like the one described
in #592, where
custom checks can be applied to any column except for ones that
are explicitly defined in the schema model class attributes

pandera/model.py

@@ -298,6 +298,8 @@ def _build_columns_index(  # pylint:disable=too-many-locals
             else:
                 dtype = annotation.arg
 
+            dtype = None if dtype is Any else dtype
+
             if annotation.origin is Series:
                 col_constructor = (
                     field.to_column if field else schema_components.Column

tests/core/test_model.py

@@ -2,7 +2,7 @@
 # pylint:disable=missing-class-docstring,missing-function-docstring,too-few-public-methods
 import re
 from decimal import Decimal  # pylint:disable=C0415
-from typing import Iterable, Optional
+from typing import Any, Iterable, Optional
 
 import pandas as pd
 import pytest
@@ -18,10 +18,11 @@ def test_to_schema() -> None:
     class Schema(pa.SchemaModel):
         a: Series[int]
         b: Series[str]
+        c: Series[Any]
         idx: Index[str]
 
     expected = pa.DataFrameSchema(
-        columns={"a": pa.Column(int), "b": pa.Column(str)},
+        columns={"a": pa.Column(int), "b": pa.Column(str), "c": pa.Column()},
         index=pa.Index(str),
     )