Without tp_clear, GC can't break cyclic reference.
It will cause memory leak when cyclic reference is
created intentionally.

PyErr_Print() will not return when the exception is a SystemExit, so
decref the __main__ module object in that case.

The Opera Browser was using a outdated command line invocation that resulted in an incorrect URL being opened in the browser when requested using the webbrowser module.

* Correct the arguments passed to the Opera Browser when opening a new URL.

When hunting memory leaks using -R 3:3, test_imap_unordered() of
test_multiprocessing leaks randomly a few memory blocks. It is a
false alarm: when testing using -R 3:20 for example, no leak is
detected.

Modify test_imap_unordered() to be closer to test_imap():

* Only test 10 numbers instead of 1000: it's a pool of 4 processes, so
  10 is enough to test at least one number per process
* Use chunksize=100 instead of chunksize=53 to mimick test_imap()

 The line-length limit is not needed because the pages appear in a separate app rather
 than on a browser tab.  It can also interact badly with the DPI setting.

Using OptiPNG 0.7.7.
Used command is: `find . -name '*.png' | xargs optipng -o7`

They can be exposed when some C API calls fail due to lack of
memory.

* Failed Py_BuildValue() could cause an assertion error in the
  following TextIOWrapper.tell().
* input_chunk could be decrefed twice in TextIOWrapper.seek()
  after failed Py_BuildValue().
* initvalue could leak in StringIO.__getstate__() after failed
  PyDict_Copy().

* `flags` is indeed deprecated, but there is a validation on its value for
  backwards compatibility reasons.  This adds mention of this in the docs.
* The docs say that `sizehint` is deprecated and ignored, but it is still
  used when `epoll_create1()` is unavailable. This adds mention of this in
  the docs.
* `sizehint=-1` is acceptable again, and is replaced with `FD_SETSIZE-1`.
  This is needed to have a default value available at the Python level,
  since `FD_SETSIZE` is not exposed to Python. (see: bpo-31938)
* Reject `sizehint=0` since it is invalid to pass on to `epoll_create()`.

The relevant tests have also been updated.

Fix passing lists and tuples of strings containing special characters
'"', '\\', '{', '}' and '\n' as options to tkinter.ttk widgets.

The 'wikipedia' example is now 'rosette', describing what it draws.
The 'penrose' print output is reduced.  The 'tree' '1024'
output is eliminated.

Increase timeouts from 10 seconds to 1 minute.

bpo-30339, bpo-33913:

* Increase timeout from 10 seconds to 1 minute in
  test_source_main_skipped_in_children source of
  test_multiprocessing_main_handling.
* Replace time.time() with time.monotonic().
* On timeout, include the duration in the error message.

test_mymanager_context() now also accepts -SIGTERM as an expected
exitcode for the manager process. The process is killed with SIGTERM
if it takes longer than 1 second to stop.

OpenProcess() creates a new handle that must be closed later.

Fix a race condition in Popen of
multiprocessing.popen_spawn_win32. The child process now duplicates
the read end of pipe instead of "stealing" it.

Previously, the read end of pipe was "stolen" by the child process,
but it leaked a handle if the child process had been terminated
before it could steal the handle from the parent process.

Tolerate a different of 50 ms, instead of just 30 ms, in
test_timeout() of multiprocessing tests. This change should fix such
test failure on Windows:

FAIL: test_timeout (test.test_multiprocessing_spawn.WithProcessesTestQueue)
Traceback (most recent call last):
  File "lib\test\_test_multiprocessing.py", line 753, in test_timeout
    self.assertGreaterEqual(delta, 0.170)
AssertionError: 0.16138982772827148 not greater than or equal to 0.17

Handle subnormal weights for choices()

Import pyshell first in htest to call SetProcessDpiAwareness on Windows
before tkinter.Tk() is called for the htest. Apparently, 'root.destroy()'
undoes a previous 'root = Tk()'. Since IDLE unittests always destroy roots,
a unittest before an htest does not require anything more to work right.
Since part of the purpose of human-viewed tests is to determine that
widgets look right, it is important that they look the same for testing
as when running IDLE.

it's better to refer to PEP with `pep` role in reStructuredText. It also links to the PEP page.