[py] Fix return type hint in get_screenshot_as_png (#10060)
[py] Fix return type hint in get_screenshot_as_png (#10060)

get_screenshot_as_png returns a bytes object but the return type hint
is set to str. This causes IDEs to throw warning when using this function
with io.BytesIO (or any other function that expects bytes object)
joshmgrant/selenium,titusfortner/selenium,joshmgrant/selenium,titusfortner/selenium,titusfortner/selenium,valfirst/selenium,titusfortner/selenium,joshmgrant/selenium,HtmlUnit/selenium,titusfortner/selenium,titusfortner/selenium,SeleniumHQ/selenium,valfirst/selenium,valfirst/selenium,titusfortner/selenium,HtmlUnit/selenium,SeleniumHQ/selenium,SeleniumHQ/selenium,valfirst/selenium,valfirst/selenium,HtmlUnit/selenium,joshmgrant/selenium,HtmlUnit/selenium,valfirst/selenium,HtmlUnit/selenium,joshmgrant/selenium,HtmlUnit/selenium,HtmlUnit/selenium,valfirst/selenium,joshmgrant/selenium,titusfortner/selenium,HtmlUnit/selenium,valfirst/selenium,titusfortner/selenium,titusfortner/selenium,joshmgrant/selenium,SeleniumHQ/selenium,joshmgrant/selenium,SeleniumHQ/selenium,valfirst/selenium,valfirst/selenium,SeleniumHQ/selenium,joshmgrant/selenium,titusfortner/selenium,SeleniumHQ/selenium,joshmgrant/selenium,SeleniumHQ/selenium,SeleniumHQ/selenium,joshmgrant/selenium,SeleniumHQ/selenium,SeleniumHQ/selenium,valfirst/selenium,HtmlUnit/selenium,HtmlUnit/selenium