reactpy v2.0.0b6

All changes from v2.0.0b5 with a few additions:

• Fix HOOK_STACK errors
• Remove key attribute from VDOM spec (Fix #1284)
• Add reactpy.html(...) shorthand notation for HTML fragments.
• Add reactpy.h shorthand alias for reactpy.html.
• Add reactpy.reactjs.component_from_npm
• Deprecate reactpy.web.*
• Move component_from_* functions into reactpy.reactjs.*
• Allow for all ReactPy and ReactJS components to be arbitrarily inserted into each other (Fix #1262)