Selenium deprecated 'switch_to_frame'

WebDriver's `switch_to_frame` has been deprecated and changed to `driver.switch_to.frame` so I updated the only place where `switch_to_frame` is used, the `get_iframe` function. All of the existing tests I ran via `make test` were unaffected by this change. There is no real behavior change other than the warning's output is now suppressed.

https://github.com/SeleniumHQ/selenium/blob/31c41534e30f0d6f38f3289f36bda347e5a8c15d/py/selenium/webdriver/remote/webdriver.py#L503