The previous tests used time.sleep() to synchronize two processes. If the host
was too slow, the test could fail.

The new tests only use one process, but they use a subprocess to:

 - have only one thread
 - have a timeout on the blocking read (select cannot be used in the test,
   select always fail with EINTR, the kernel doesn't restart it)
 - not touch signal handling of the parent process

number of processes if negative or null.

processes if negative or null.

Use os.access(path, os.X_OK) instead of (os.stat(path).st_mode & 0o111 != 0),
and ignore the test on Windows.

(UnicodeEncodeError).

module name anymore, only work on unicode strings. Therefore it doesn't
truncate module names with embedded NUL characters, or fail if the module name
contains surrogate characters (UTF-8 encoder fails on a surrogate character).

Patch written by Alexander Belopolsky.

encoding using _Py_char2wchar() instead of mbstowcs() to store undecodable
bytes as surrogates characters (PEP 383) instead of ignoring silently
the PYTHONPATH variable.

This prevents the assignment of __class__ in the class body from breaking
super. (Although a determined person could do locals()["@__class__"] = 4)

Original patch by Vinay Sajip on #11637.