BUG: Regression in assert_frame_equal check_dtype for extension dtypes
Consider this small example of two DataFrames, one with an Int64 extension dtype, the other with the same values but object dtype:

```
df1 = pd.DataFrame({'a': pd.array([1, 2, 3], dtype="Int64")}) 
df2 = df1.astype(object)   
```

With pandas 1.0.1, this passes `assert_frame_equal` with the `check_dtype=False`:

```
In [5]: pd.testing.assert_frame_equal(df1, df2)  
...
Attribute "dtype" are different
[left]:  Int64
[right]: object

In [6]: pd.testing.assert_frame_equal(df1, df2, check_dtype=False)  
```

but with master (since https://github.com/pandas-dev/pandas/pull/32570, see my comment there, cc @jbrockmendel), this fails:

```
In [2]: pd.testing.assert_frame_equal(df1, df2, check_dtype=False)   
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-2-d2d792202db1> in <module>
----> 1 pd.testing.assert_frame_equal(df1, df2, check_dtype=False)

~/scipy/pandas/pandas/_testing.py in assert_frame_equal(left, right, check_dtype, check_index_type, check_column_type, check_frame_type, check_less_precise, check_names, by_blocks, check_exact, check_datetimelike_compat, check_categorical, check_like, obj)
   1378                 check_datetimelike_compat=check_datetimelike_compat,
   1379                 check_categorical=check_categorical,
-> 1380                 obj=f'{obj}.iloc[:, {i}] (column name="{col}")',
   1381             )
   1382 

~/scipy/pandas/pandas/_testing.py in assert_series_equal(left, right, check_dtype, check_index_type, check_series_type, check_less_precise, check_names, check_exact, check_datetimelike_compat, check_categorical, check_category_order, obj)
   1177         )
   1178     elif is_extension_array_dtype(left.dtype) or is_extension_array_dtype(right.dtype):
-> 1179         assert_extension_array_equal(left._values, right._values)
   1180     elif needs_i8_conversion(left.dtype) or needs_i8_conversion(right.dtype):
   1181         # DatetimeArray or TimedeltaArray

~/scipy/pandas/pandas/_testing.py in assert_extension_array_equal(left, right, check_dtype, check_less_precise, check_exact)
   1017     """
   1018     assert isinstance(left, ExtensionArray), "left is not an ExtensionArray"
-> 1019     assert isinstance(right, ExtensionArray), "right is not an ExtensionArray"
   1020     if check_dtype:
   1021         assert_attr_equal("dtype", left, right, obj="ExtensionArray")

AssertionError: right is not an ExtensionArray
```