Return 304 response if file was not modified.

* change LBYL key lookup to dict.setdefault

The ``results`` was constructed as a defaultdict and we could simply
delete the check ``if key not in results``. However, I think it's safer
to use dict.setdefault as I'm not sure whether the caller expects a
regular dict or defaultdict.

* add name to the acknowledgements file

* use defaultdict to make the key-lookup cleaner

Revert "Minor factoring:  move redundant resize scaling logic into the resize function."

This reverts commit 48973002.

This reverts commit b94d7fd4.

the original logic was just comparing the network address
but this is wrong because if the network address is equal then
we need to compare the ip address for breaking the tie

add more ip_interface comparison tests

Also uncomment and fix a path test.

objects when pass out of bound fold argument.

Fix MemoryError caused by moving around code in PR #886; nbytes was sometimes used unitinitalized (in non-debug builds, when use_calloc was false and elsize was 0).

Make a non-Py_DEBUG, asserts-enabled build of CPython possible. This means
making sure helper functions are defined when NDEBUG is not defined, not
just when Py_DEBUG is defined.

Also fix a division-by-zero in obmalloc.c that went unnoticed because in Py_DEBUG mode, elsize is never zero.

Changed test code to suppress a compiler warning, while taking care to avoid the code being optimized out by the compiler.

s/keys and elements/keys and values/

Remove catching OSError in ftphandler test. Only URLError is raised in urllib.request module. (#918)

Fix the use of recursion in itertools.chain.from_iterable. Using recursion
is unnecessary, and can easily cause stack overflows, especially when
building in low optimization modes or with Py_DEBUG enabled.

when pass indices of wrong type.

Element.getiterator() and the html parameter of XMLParser() were
deprecated only in the documentation (since Python 3.2 and 3.4 correspondintly).
Now using them emits a deprecation warning.

* Don’t need check_warnings any more.

* test_normalization fails if download fails

bpo-29887. The test is still skipped if "-u urlfetch" option is not
passed to regrtest (python3 -m test -u urlfetch test_normalization).

* Fix ResourceWarning in test_normalization

bpo-29887: Fix ResourceWarning in test_normalization if tests are
interrupted by CTRL+c.

ValueError always is raised rather than OverflowError for negative counts.
Shifting zero with non-negative count always returns zero.

if pass `accept={int, NoneType}`.

os.fwalk() is sped up by 2 times by using os.scandir().

unlucky Unicode characters.

an exception raised at the very first of an iterable would cause pools behave abnormally
(swallow the exception or hang)