ExtensionDtype is missing hasobject
#### Problem description

For better compatibility with numpy dtypes, `hasobject` could be added to `ExtensionDtype`. We use it to determine if columns contain Python objects or not, but it does not work with sparse column types which are extending `ExtensionDtype`.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.18.0-20-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.25.0.dev0+610.gd2beaf3c8
pytest: None
pip: 18.1
setuptools: 40.7.1
Cython: 0.29.7
numpy: 1.15.4
scipy: 1.2.0
pyarrow: 0.13.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
