Fix typing.Union checks with Python 3.7

doc/changelog.asciidoc

@@ -76,6 +76,7 @@ Fixed
 - QtWebEngine: Qt download objects are now cleaned up properly when a download
   is removed.
 - Suspended pages now should always load the correct page when being un-suspended.
+- Compatibility with Python 3.7
 
 Removed
 ~~~~~~~

qutebrowser/commands/command.py

@@ -394,10 +394,8 @@ class Command:
         if isinstance(typ, tuple):
             raise TypeError("{}: Legacy tuple type annotation!".format(
                 self.name))
-        elif type(typ) is type(typing.Union):  # noqa: E721
+        elif getattr(typ, '__origin__', None) is typing.Union:
             # this is... slightly evil, I know
-            # We also can't use isinstance here because typing.Union doesn't
-            # support that.
             # pylint: disable=no-member,useless-suppression
             try:
                 types = list(typ.__args__)