bpo-41100: Update Whatsnew and installer ReadME for 3.9.1 (GH-23472)

Author: ned-deily

Doc/whatsnew/3.9.rst

@@ -1482,4 +1482,18 @@ and to match the behavior of static type checkers specified in the PEP.
         File "<stdin>", line 1, in <module>
       TypeError: unhashable type: 'set'
 
-(Contributed by Yurii Karabas in :issue:`42345`.)
+(Contributed by Yurii Karabas in :issue:`42345`.)
+
+macOS 11.0 (Big Sur) and Apple Silicon Mac support
+--------------------------------------------------
+
+As of 3.9.1, Python now fully supports building and running on macOS 11.0
+(Big Sur) and on Apple Silicon Macs (based on the ``ARM64`` architecture).
+A new universal build variant, ``universal2``, is now available to natively
+support both ``ARM64`` and ``Intel 64`` in one set of executables. Binaries
+can also now be built on current versions of macOS to be deployed on a range
+of older macOS versions (tested to 10.9) while making some newer OS
+functions and options conditionally available based on the operating system
+version in use at runtime ("weaklinking").
+
+(Contributed by Ronald Oussoren and Lawrence D'Anna in :issue:`41100`.)

Mac/BuildScript/resources/ReadMe.rtf

@@ -3,7 +3,7 @@
 \f3\fmodern\fcharset0 CourierNewPSMT;}
 {\colortbl;\red255\green255\blue255;}
 {\*\expandedcolortbl;;}
-\margl1440\margr1440\vieww13380\viewh14600\viewkind0
+\margl1440\margr1440\vieww13380\viewh14580\viewkind0
 \pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\pardirnatural\partightenfactor0
 
 \f0\fs24 \cf0 This package will install Python $FULL_VERSION for macOS $MACOSX_DEPLOYMENT_TARGET for the following architecture(s): $ARCHITECTURES.\
@@ -50,7 +50,21 @@ Due to new security checks on macOS 10.15 Catalina, when launching IDLE macOS ma
 \f0\b0  button to proceed.\
 \
 
-\f1\b \ul Other changes\
+\f1\b \ul macOS 11.0 (Big Sur) and Apple Silicon Mac support [new in 3.9.1]\
+
+\f0\b0 \ulnone \
+As of 2020-11, macOS 11.0 (Big Sur) is the latest release of macOS and one of its major features is the support of new Apple Silicon Macs that are based on the ARM64 CPU architecture specification rather than the Intel 64 (x86_64) architecture used previously. There are other changes in Big Sur that affect Python operation regardless of CPU architecture. As of 3.9.1, Python binaries from python.org fully support Big Sur. \
+\
+python.org binaries for macOS have been provided via a downloadable installer that supports the Intel 64 architecture on macOS 10.9 and newer.  This installer variant remains the default download for 3.9.1;  it will install and run on all Macs that run macOS 10.9 or later, including 11.0 (Big Sur). This variant should run transparently on new Apple Silicon Macs using Apple's Rosetta 2 emulation. \
+\
+Beginning with 3.9.1, we also provide a new "universal2" installer variant that provides universal binaries for both ARM64 and Intel 64 architectures and is also supported on all Macs that support macOS 10.9 or later.  Some of the advantages of the new installer variant: native ARM64 code on Apple Silicon Macs should run significantly faster than Rosetta2-emulated code; some operating system functions and options introduced in macOS releases since 10.9 are now exposed when available (primarily in the os module); binary wheels built for use with the current 10.9 variant *should* also work with the new variant; the new installer variant includes Tcl/Tk 8.6.10 rather than 8.6.8.\
+\
+Because of the scope of changes needed to fully support 11.0 and Apple Silicon Macs, the new "universal2" variant should be considered 
+\f2\i experimental
+\f0\i0  in the 3.9.1 release. You may need to upgrade third-party components, like pip, to later versions once they are released. You may experience differences in behavior in IDLE and other Tk-based applications due to using the newer version of Tk.  As always, if you encounter problems when using this installer variant, please check {\field{\*\fldinst{HYPERLINK "https://bugs.python.org"}}{\fldrslt https://bugs.python.org}} for existing reports and for opening new issues.\
+
+\f1\b \ul \
+Other changes\
 
 \f0\b0 \ulnone \
 For other changes in this release, see the