Fix failing test on in-place division

I've added support for the opcodes INPLACE_FLOOR_DIVIDE and INPLACE_TRUE_DIVIDE.  As far as I can tell, Python3 should never emit an INPLACE_DIVIDE (the opcode isn't defined in opcodes.py).  In pyvm2.py, I've added the future division import so that the same code ("/=") will behave the same way under py2 and py3.  I think this makes the tests neater.

I wrote some tests for this functionality, but they're somewhat ugly and I'm not sure if they're hitting the right level of abstraction, so please let me know how they could be better.
Fix in-place division

I've added support for the opcodes INPLACE_FLOOR_DIVIDE and INPLACE_TRUE_DIVIDE.  As far as I can tell, Python3 should never emit an INPLACE_DIVIDE (the opcode isn't defined in opcodes.py).  In pyvm2.py, I've added the future division import so that the same code ("/=") will behave the same way under py2 and py3.  I think this makes the tests neater.

I wrote some tests for this functionality, but they're somewhat ugly and I'm not sure if they're hitting the right level of abstraction, so please let me know how they could be better.
Add an authors file.
Update tests with isinstance and non-evenly-dividing tests.
Merge pull request #1 from akaptur/division-fix

Fix in-place division
Thanks, Allison!
Add pop_block method
intermediate steps towards with fix
Merge branch 'master' of https://github.com/nedbat/byterun

Conflicts:
	tests/test_basic.py