Add "contact_us" model field to the AgencyChain resource

[marz619]

The new field contains three attributes; email, phone_number, and website_url; which provide corresponding "contact us" values.

This will be released as version 2.36.0 (Release: https://pypi.org/project/gapipy/2.36.0/)

NOTE: This field will be None (null) if all of the contained fields have None values.

Usage:

In [1]: from gapipy import Client

In [2]: gapi = Client(application_key="test_abcd...")

In [3]: ac = gapi.agency_chains.get("a1234000000ABCDEF4")  # null contact_us

In [4]: ac.contact_us is None
Out [4]: True

In [5]: ac = gapi.agency_chains.get("a1234000000ABCDEF5")  # non-null contact_us

In [6]: ac.contact_us
Out[6]: <gapipy.resources.booking.agency_chain.ContactUs at 0x1053a9760>

In [7]: ac.contact_us.website_url
Out[7]: 'https://www.example.com/contact-us'

In [8]: ac._raw_data['contact_us']
Out[8]:
{'email': None,
 'phone_number': None,
 'website_url': 'https://www.example.com/contact-us'}