Binder isn't cleared correctly with --scripts-are-modules

[rwbarton]

Consider these two files:

# sam1.py
f = 'a'
f = 'b'

# sam2.py
f = 1
reveal_type(f)

Running python3 -m mypy --scripts-are-modules sam2.py sam1.py (note the order; it seems mypy processes the files in reverse order) produces

sam2.py:2: error: Revealed type is 'builtins.None'

I'm pretty sure this is due to old typestate left in the binder, because deleting the second assignment in sam1.py makes the problem go away. (And we don't populate the binder on the initial assignment to a variable: #2008.)

[rwbarton]

Oops, my bad. I was trying to reproduce an issue in Zulip, which is on mypy 0.4.5, so I had that version of mypy checked out. It appears that I already fixed this somehow in 3b92e79.