Also promote notices from distutils doc to deprecation boxes.

This removes the need to call flush manually in each test, except when
testing code that creates warning without checking them.

This updates the user guide to refer to Packaging instead of Distutils.
Some files still require an update.

document.

_check_dump_tracebacks_later() to avoid failures if the system is slow.

the file descriptor of a pipe closed in the parent process is valid in the
child process according to fstat(), but the mode of the file descriptor is
invalid, and read or write raise an error.

test.support.requires_mac_ver() is now a decorator, as suggested by Ezio
Melotti, and its docstring is fixed (linux_version => mac_ver).

descriptor of a pipe closed in the parent process is valid in the child process
according to fstat(), but the mode of the file descriptor is invalid, and read
or write raise an error.

Add also requires_mac_ver() decorator to test.support.

Add also linux_version() to __all__.

if the constructor has failed, e.g. because of an undeclared keyword argument.
Patch written by Oleg Oshmyan.

constructor has failed, e.g. because of an undeclared keyword argument. Patch
written by Oleg Oshmyan.

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

io.BufferedReader._peek_unlocked(). Patch written by John O'Connor.