bpo-10381, bpo-32403: What's new entries for changes to datetime (pythongh-5814)

* Add What's New entry for addition of datetime.timezone to the C API

Closes bpo-10381

* Add what's new entry for date and datetime optimizations

Closes bpo-32403

Author: pganssle

Doc/whatsnew/3.7.rst

@@ -886,6 +886,11 @@ Optimizations
   start-up time by up to 10%. (Contributed by Ivan Levkivskyi and INADA Naoki
   in :issue:`31333`)
 
+* Significant speed improvements to alternate constructors for
+  :class:`datetime.date` and :class:`datetime.datetime` by using fast-path
+  constructors when not constructing subclasses. (Contributed by Paul Ganssle
+  in :issue:`32403`)
+
 Build and C API Changes
 =======================
 
@@ -936,6 +941,11 @@ Build and C API Changes
 * Support for building ``--without-threads`` is removed.
   (Contributed by Antoine Pitrou in :issue:`31370`.).
 
+* Added C API support for timezones with timezone constructors
+  :c:func:`PyTimeZone_FromOffset` and :c:func:`PyTimeZone_FromOffsetAndName`,
+  and access to the UTC singleton with :c:data:`PyDateTime_TimeZone_UTC`.
+  Contributed by Paul Ganssle in :issue:`10381`.
+
 
 Other CPython Implementation Changes
 ====================================