Don't assume that for loop body index variable is always set

[JukkaL]

Fixes #18629. Fixes #16321. Fixes #8637.

[github-actions]

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

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/assertion/rewrite.py:731: error: Item "None" of "Optional[stmt]" has no attribute "lineno"  [union-attr]

kopf (https://github.com/nolar/kopf): 1.83x faster (40.2s -> 22.0s in a single noisy sample)

core (https://github.com/home-assistant/core)
+ homeassistant/components/blueprint/importer.py:223: error: Value of type "Any | None" is not indexable  [index]

pyodide (https://github.com/pyodide/pyodide)
+ src/py/pyodide/code.py:45: error: Unsupported operand types for + ("None" and "int")  [operator]
+ src/py/pyodide/code.py:45: note: Left operand is of type "int | None"

[JukkaL]

All the new mypy_primer errors look right.

[ilevkivskyi]

@JukkaL I think this may also fix #8637 (an old high priority bug), could you please double-check?

[JukkaL]

I think this may also fix #8637 (an old high priority bug)

Yes, it is fixed!