Bug 1839342 - Don't use typing.Literal, r=webdriver-reviewers

This doesn't exist in Python 3.7, which is our minimum supported version.

Differential Revision: https://phabricator.services.mozilla.com/D181456

testing/web-platform/tests/webdriver/tests/support/inline.py

@@ -1,6 +1,6 @@
 """Helpers for inlining extracts of documents in tests."""
 
-from typing import Literal, Optional
+from typing import Optional
 from urllib.parse import urlencode
 
 
@@ -27,7 +27,7 @@ MIME_TYPES = {
 
 
 def build_inline(build_url, src,
-                 doctype: Literal["html", "xhtml", "xml"] = "html",
+                 doctype: str = "html",
                  mime: Optional[str] = None, charset: Optional[str] = None,
                  parameters = None, **kwargs):
     if mime is None: