Using the ternary here in the key name isn't particularly readable. How about pulling the key out into a variable and using a plain conditional?

if self._is_email(username):
    login_key = 'email'
else:
    login_key = 'login'

rsp, headers = self._json_post(
    url=self._build_api_url(hosting_url, 'session'),
    fields={
        login_key: username,
        'password': password,
    })


You could maybe even replace the conditional with try/except and get rid of the _is_email method.

This shouldn't be necessary. Just defining login_key inside the conditional is fine.