Fix and add additional tests

[jepler]

Fixes:

• Multiple function decorators
• Multiple slices (existing failing test)
• Multiline f-string (existing failing test)
• try/except/else/finally
• Annotated assignment
• Async generators

Add test for:

• reported problem with double quotes in string (did not reproduce reported problem)

I think this closes #22 by adding tests for all reported cases.
It also closes #21 by explicitly reading README.md in UTF-8 encoding.

[jepler]

This passes locally, where I tested with 3.9 only. In 2022, what range of python versions do you want to support?

[jepler]

This is now green on travis in my fork, testing python 2.7 through 3.9.

[jepler]

Closing this as I'm no longer trying to use astmonkey.