Add support for Python 3.9

[jansel]

The process for adding support for Python 3.9 is as follows.

First, examine the 10 new bytecodes in 3.9:

RERAISE
raising exceptions is currently not supported and breaks the graph. So this can just call unimplemented() for now. See issue pytorch/pytorch#93720 for more on exceptions.

WITH_EXCEPT_START
might affect support for with no_grad(): and related ops

LOAD_ASSERTION_ERROR
LIST_TO_TUPLE
LIST_EXTEND
SET_UPDATE
DICT_UPDATE
DICT_MERGE
IS_OP
CONTAINS_OP
these are all simple aliases for things TorchDynamo already supports and should be easy to handle

Next, update the versions supported in setup.py so the build works.

Next, iteratively add support for ops and fix issues until all tests pass. pytest tests to run tests.

Next, iteratively fix issues in ./torchbench.py until all models pass and match the coverage of Python 3.8

[jansel]

Looks like 3.9 also added
_PyInterpreterState_SetEvalFrameFunc https://docs.python.org/dev/c-api/init.html#c._PyInterpreterState_SetEvalFrameFunc
and added a new PyThreadState *tstate arg to the the eval_frame function:
https://docs.python.org/dev/c-api/init.html#c._PyFrameEvalFunction

Both of those will require changes to _eval_frame.c

[jansel]

@hsrussell is taking a look at this one

[jansel]

def LIST_TO_TUPLE(self, inst):
   self.push(BuiltinVariable(tuple).call_function(self, [self.pop()], {}))

[yanboliang]

Fixed by #154