default namespace in find method

[cukiernick]

Currently namespaces type in find method is defined as _StrictNSMap = Mapping[str, str], which causes mypy error being reported when default namespace (None key) is used. Is it correct?
lxml supports 2 ways of specifying the default namespace: None and an empty string (""). Because nsmap attribute of Element shows the default namespace under None key, I'd expect find to also be allowed to use this format.

Small snippet from ipython:

In [13]: z = lxml.etree.XML("<a><b xmlns='foo'/></a>")

In [14]: z.find('./b')

In [15]: z.find('./b', namespaces={None: "foo"})
Out[15]: <Element {foo}b at 0x7f6df2c0e480>

In [16]: z.find('./b', namespaces={"": "foo"})
Out[16]: <Element {foo}b at 0x7f6df2c0e480>

In [17]: z.find('./b', namespaces={"": "foo"}).nsmap
Out[17]: {None: 'foo'}

[abelcheung]

You're correct, I have been overzealous with the namespaces dict key change. Originally it is intended to prevent silly usage like find('*', namespaces={None: "foo", "": "bar"}), but such corner cases hurts more general usage. Will fix it soon.