Not only sys.executable. Sources of non-ASCII paths:

* sys.executable
* __file__ of the stdlib or test modules
* the current working directory
* the temporary directory

The last one is the most common in these failures.

Tests fail when a non-ASCII path is written to the stdout or a file with the default encoding (which differs from the filesystem encoding) and then read with implying:

* the ASCII encoding
* the UTF-8 encoding
* the filesystem encoding

Fixing tests is not enough, because it is often an issue of scripts which write paths to the stdout. This problem does not have simple and general solution.