A first stab. We also have to split _TextMode and _BinaryMode, similar to how they are split in builtins.pyi. We probably don't need to handle the read case, as a read-only temp file makes no sense. This could even help catch further bugs.

Depends on #10233.

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas) got 1.05x slower (126.4s -> 133.3s)

This is the last prerequisite for trying #10229 again.