https://bugs.python.org/issue38155

This fixes an inconsistency between the Python and C implementations of
the datetime module. The pure python version of the code was not
accepting offsets greater than 23:59 but less than 24:00. This is an
accidental legacy of the original implementation, which was put in place
before tzinfo allowed sub-minute time zone offsets.

GH-14878

There was a discrepancy between the Python and C implementations.

Add singletons ALWAYS_EQ, LARGEST and SMALLEST in test.support
to test mixed type comparison.

Returns NotImplemented for timedelta and time in __eq__ for different types in Python implementation, which matches the C implementation.

This also adds tests to enforce that these objects will fall back to the right hand side's __eq__ and/or __ne__ implementation.

bpo-37579

This commit implements the first version of date.fromisocalendar, the
inverse function for date.isocalendar.

Deprecate using the __int__() method in implicit conversions of Python
numbers to C integers.

* Make timedelta return subclass types

Previously timedelta would always return the `date` and `datetime`
types, regardless of what it is added to. This makes it return
an object of the type it was added to.

* Add tests for timedelta arithmetic on subclasses

* Make pure python timedelta return subclass types

* Add test for fromtimestamp with tz argument

* Add tests for subclass behavior in now

* Add news entry.

Fixes:
bpo-32417
bpo-35364

* More descriptive variable names in tests

Addresses Victor's comments

encoding='latin1' should be used for successful decoding.

* Use _PyUnicode_Copy in sanitize_isoformat_str

* Use repr in fromisoformat error message

This reverses commit 67b74a98b2 per Serhiy Storchaka's suggestion:

     I suggested to use %R in the error message because including the raw
     string can be confusing in the case of empty string, or string
     containing trailing whitespaces, invisible or unprintable characters.

We agree that it is better to change both the C and pure Python versions
to use repr.

* Retain non-sanitized dtstr for error printing

This does not create an extra string, it just holds on to a reference to
the original input string for purposes of creating the error message.

* PEP 7 fixes to from_isoformat

* Separate handling of Unicode and other errors

In the initial implementation, errors other than encoding errors would
both raise an error indicating an invalid format, which would not be
true for errors like MemoryError.

* Drop needs_decref from _sanitize_isoformat_str

Instead _sanitize_isoformat_str returns a new reference, even to the
original string.

On Windows, passing a negative value to local results in an OSError because localtime_s on Windows does not support negative timestamps. Unfortunately this means that fold detection for timestamps between 0 and max_fold_seconds will result in this OSError since we subtract max_fold_seconds from the timestamp to detect a fold. However, since we know there haven't been any folds in the interval [0, max_fold_seconds) in any timezone, we can hackily just forego fold detection for this time range on Windows.