This change makes configure check for
- the existence of a hg repository
- the hg executable itself
- the python executable

Running $(srcdir)/Parser/asdl_c.py (i.e. ASDLGEN) will fail if any of
the above prerequisites is missing, so we now disable it instead.

closes #12225

Original patch by Nicolas Estibals.  My tweaks to the patch were mostly
style/cosmetic, and adding more tests.

freed by the garbage collector while the Heap lock is held.

the garbage collector while the Heap lock is held.

Move also the "ready" trigger after the installation of the signal handler and
the call to siginterrupt().

Use a timeout of 5 seconds instead of 3. Two seconds are supposed to be enough,
but some of our buildbots are really slow (especially the FreeBSD 6 VM).

Backport commits 968b9ff9a059 and aff0a7b0cb12 from the default branch to 3.2
branch. Extract of the changelog messages:

"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"

and

"Add a basic synchronization code between the child and the parent processes:
the child writes "ready" to stdout."

I replaced .communicate(timeout=3.0) by an explicit waiting loop using
Popen.poll().

with known OS bugs

Share the list of platforms with known OS bugs with other tests. Patch written
by Charles-François Natali.

TemporaryFileTests has tests for os.tempnam() and os.tmpfile(), functions
removed from Python 3.

Move fdopen() tests to the FileTests testcase to test fdopen() on a file
descriptor, not on a directory descriptor (which raises an error on Windows).

py-list displays the error. py-bt ignores the error (the filename and line
number is already displayed).

The testcase was never executed, it's now fixed.

needed.

Remove also an unused variable (blank) in test_threading.

UTF-8 instead of the locale encoding if the encoding is not specified. It now
also opens XML files for the parser in binary mode instead of the text mode to
avoid encoding issues.

avoid encoding issues (it used the locale encoding, whereas UTF-8 should be).

Remove also an unused import (warnings).

read the Python script from the right encoding.

encoding cookie, instead of opening it in UTF-8.

.truncate(0) doesn't rewind.

anymore

regrtest doesn't check that a test doesn't output anything anymore.

regrtest doesn't check that tests doesn't write something to stdout anymore.

Don't replace sys.stdout by the original sys.stdout to be able to capture the
output for regrtest -W.

If verbose is False, the output is empty. Fix also a typo in a variable name.

 * Don't force verbose to True with option -W
 * Rename rerun_failed variable to output_on_failure

a io.StringIO object by regrtest (which has no file descriptor).

the output and displays it on failure instead. regrtest -v doesn't print the
error twice anymore if there is only one error.