setup: include vendored 3rd party type stubs

While pytype may not support PEP 561, PyCharm does, so having the stubs
available means it can determine `foo = attr.ib(type=int)` means `foo` is an
int.  This only applies when using Mercurial as a library, like with TortoiseHg
development- PyCharm is already smart enough to use the *.pyi files in the
Mercurial source tree when hacking on Mercurial itself.

I left the mercurial.cext stubs out because it seems very low level, that 3rd
parties shouldn't be using directly.

setup.py

@@ -1607,6 +1607,10 b' packagedata = {'
     'mercurial.helptext.internals': [
         '*.txt',
     ],
+    'mercurial.thirdparty.attr': [
+        '*.pyi',
+        'py.typed',
+    ],
 }