Give "as" variables in with statements separate scopes

[JukkaL]

Use renaming to allow two with statements to define the same variable
using incompatible types. The implementation is pretty simple-minded
and doesn't work in cases where there is even a hint of potential
ambiguity about which variant of a variable a reference targets, typically
due to functions.

This could be generalized to for statements in the future.

Fixes #12246.

[github-actions]

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

bandersnatch (https://github.com/pypa/bandersnatch)
+ src/bandersnatch/tests/test_utils.py:74: error: Unused "type: ignore" comment
+ src/bandersnatch/tests/test_utils.py:88: error: Unused "type: ignore" comment
+ src/bandersnatch/tests/plugins/test_storage_plugins.py:686: error: Unused "type: ignore" comment
+ src/bandersnatch/tests/plugins/test_storage_plugins.py:696: error: Unused "type: ignore" comment

zulip (https://github.com/zulip/zulip)
- tools/setup/generate_integration_bots_avatars.py:54: error: Incompatible types in assignment (expression has type "BufferedWriter", variable has type "BufferedReader")  [assignment]

pip (https://github.com/pypa/pip)
- src/pip/_internal/req/req_uninstall.py:623: error: Incompatible types in assignment (expression has type "BufferedWriter", variable has type "BufferedReader")
- src/pip/_internal/operations/install/wheel.py:107: error: Incompatible types in assignment (expression has type "BufferedWriter", variable has type "BufferedReader")

pyinstrument (https://github.com/joerick/pyinstrument)
- pyinstrument/renderers/html.py:79: error: Incompatible types in assignment (expression has type "StreamReaderWriter", variable has type "StreamWriter")