support `itemscope` as a boolean attribute

[jvanasco]

itemscope should be supported as a boolean attribute.

for reference, schema.org has been pushing it as such (https://schema.org/docs/gs.html#microdata_itemscope_itemtype)

running something through the Tokenizer/Sanitizer will set it to an empty attribute

input: <article itemscope>
output: <article itemscope="">

it looks like this can be handled by editing constants.py and doing the following to enable it on all tags:

booleanAttributes = {
-    "": frozenset(["irrelevant"]),
+    "": frozenset(["irrelevant", "itemscope"]),

[gsnedders]

Should probably just check the current list of boolean attributes in the spec, really. We're probably missing others too.

[willkg]

I think this bug is fixed now, but if there are still outstanding things in this bug, please add a comment about what still needs to be done.

Thank you for reporting!

[jvanasco]

thanks for finding/merging my old PR!