and better generated code (on both GCC and CLang).

and for better code generation.

* Add comment to the maxlen structure entry about the meaning of maxlen == -1
* Factor-out code common to deque_append(left) and deque_extend(left)
* Factor inner-loop in deque_clear() to use only 1 test per loop instead of 2
* Tighten inner-loops for deque_item() and deque_ass_item() so that the
  compiler can combine the decrement and test into a single step.

* Write one import per line
* Sort imports by name
* Add an empty line: 2 empty lines between code blocks at the module level (PEP 8)

with no known parent package.

Previously SystemError was raised if the parent package didn't exist
(e.g., __package__ was set to '').
Thanks to Florent Xicluna and Yongzhi Pan for reporting the issue.

In a previous change, __spec__.parent was prioritized over
__package__. That is a backwards-compatibility break, but we do
eventually want __spec__ to be the ground truth for module details. So
this change reverts the change in semantics and instead raises an
ImportWarning when __package__ != __spec__.parent to give people time
to adjust to using spec objects.

Issue #25907: Use {% trans %} tags in HTML templates to ease the translation of
the documentation. The tag comes from Jinja templating system, used by Sphinx.

Patch written by Julien Palard.

Python with more options.

Issue #26161: Use Py_uintptr_t instead of void* for atomic pointers in
pyatomic.h. Use atomic_uintptr_t when <stdatomic.h> is used.

Using void* causes compilation warnings depending on which implementation of
atomic types is used.

Issue #25843: When compiling code, don't merge constants if they are equal but
have a different types. For example, "f1, f2 = lambda: 1, lambda: 1.0" is now
correctly compiled to two different functions: f1() returns 1 (int) and f2()
returns 1.0 (int), even if 1 and 1.0 are equal.

Add a new _PyCode_ConstantKey() private function.

Issue #25843: When compiling code, don't merge constants if they are equal but
have a different types. For example, "f1, f2 = lambda: 1, lambda: 1.0" is now
correctly compiled to two different functions: f1() returns 1 (int) and f2()
returns 1.0 (int), even if 1 and 1.0 are equal.

Add a new _PyCode_ConstantKey() private function.

Issue #26099: The site module now writes an error into stderr if sitecustomize
module can be imported but executing the module raise an ImportError. Same
change for usercustomize.

minor clarification on Zipfile 'x' mode - exclusive creation of a file.

(Based on the feedback from docs@python.org list)

A single call to Pool.apply_async() will create only one process. To use all
of the pool's processes, it should be invoked multiple times:

    with Pool(processes=4) as pool:
        results = [pool.apply_async(func, ()) for i in range(4)]

Patch by Davin Potts.

A single call to Pool.apply_async() will create only one process. To use all
of the pool's processes, it should be invoked multiple times:

    with Pool(processes=4) as pool:
        results = [pool.apply_async(func, ()) for i in range(4)]

Patch by Davin Potts.

issue25909 - Correct the documentation of PyMapping_Items, PyMapping_Keys and
PyMapping_Values in Include/abstract.h and Doc/c-api/mapping.rst.

Patch contributed by Sonali Gupta.

PyMapping_Values in Include/abstract.h and Doc/c-api/mapping.rst.

Patch contributed by Sonali Gupta.

Double parenthesis

This change helps to ignore text of PSF, BEOPEN.com and CNRI licenses when
translating the documentation. Patch written by Julien Palard who is
translating Python 3.5 doc to french. Text of other licenses already used
preformatted format.