arrow

ARROW-2603: [Python] Allow date and datetime subclassing

When converting a pandas dataframe holding subclasses of date/datetime objects, arrow raises an ArrowInvalid exception

```
import pandas as pd
import pyarrow as pa
import datetime

classMyDate(datetime.date):
    pass

date_array = [MyDate(2000, 1, 1)]
df = pd.DataFrame({"date": pd.Series(date_array, dtype=object)})

table = pa.Table.from_pandas(df)
```

Author: fjetter <fjetter@users.noreply.github.com>

Closes #2061 from fjetter/ARROW_2603/datetime_subclassing and squashes the following commits:

cd76b3e0 <fjetter> python: Allow date and datetime subclassing
79e420ad <fjetter> convert_pandas: cast datetime subclass explicitly during tests
927bae3d <fjetter> fix tests
11aad905 <fjetter> python: Allow date and datetime subclassing