This change ports pyenv and Python patches to 3.7.12 to enable the
`ctypes `and `decimal` modules to compile.

While Python 3.7.12 itself compiles on arm64/M1, both of these modules
fail to compile, due to missing support for locating system libffi and
due to architecture gate-keeping. These issues have been fixed in newer
releases of Python, and in other pyenv patch bundles.

The following patches are provided:

1. `0001-Port-ctypes-and-system-libffi-patches-for-arm64-macO.patch` -
   Fixes system `ffi.h`/`libffi` path determination and usage and
   enables calling of variadic functions, fixing ctypes support
   (consolidated port of existing pyenv patches for 2.7.18 that iterate
   on this logic).

2. `0002-bpo-41100-fix-_decimal-for-arm64-Mac-OS-GH-21228.patch` -
   Adds arm64 to the list of allowable architectures for the
   `decimal` module (port of Python patch introduced in 3.8.10).

This change ports several established patches to the Python 3.6.15
build, enabling compilation on arm64/Apple M1 architectures:

1. `0001-Detect-arm64-in-configure.patch` -
   Updates configure to detect arm64 architectures (port of an existing
   pyenv patch for 2.7.18).

2. `0002-bpo-36231-Support-building-on-macOS-without-usr-incl.patch` -
   Adds macOS SDK root computation logic for determining include paths
   (port of existing Python patches introduced in 2.7.17 and 3.7.4).

3. `0003-Fix-macOS-_tkinter-use-of-Tck-Tk-in-Library-Framewor.patch` -
   Fixes Tcl/Tk support on macOS (port of an existing pyenv patch
   for 2.7.18).

4. `0004-Port-ctypes-and-system-libffi-patches-for-arm64-macO.patch` -
   Fixes system `ffi.h`/`libffi` path determination and usage and
   enables calling of variadic functions, fixing ctypes support
   (consolidated port of existing pyenv patches for 2.7.18 that iterate
   on this logic).

5. `0005-BPO-41100-Support-macOS-11-when-building-GH-21113.patch` -
   Updates Darwin version checks to handle macOS 11's major version
   bump (port of Python patches introduced in 3.7.0 and 3.9.0).

6. `0006-bpo-41100-fix-_decimal-for-arm64-Mac-OS-GH-21228.patch` -
   Adds arm64 to the list of allowable architectures for the
   `decimal` module (port of Python patch introduced in 3.8.10).

* Use verify_py39 since anaconda3-2021.11 is using python-3.9 according to release notes

Fix 3.6.15 build on macOS

[source](https://www.anaconda.com/blog/anaconda-individual-edition-2021-11#:~:text=Update%20on%20macOS%20Apple%20M1%20Support)

`which` has been deprecated in Debian's debianutils 5.0+
They suggest using `type` and `command` as replacements:
3a8dd10b45 ,
610a65c6ef

The issue has been resolved in ed38af6409
but the bounty didn't trigger -- allegedly 'cuz it's not a PR.

Bountysource support replied "we're looking" but never did anything
so I'm going to try to trigger it via a dummy PR.

* Fix "PKG_PROG_PKG_CONFIG: command not found"

Apply f146ca36f8
* bpo-45350: Rerun autoreconf with the pkg-config macros (GH-28707)

This is needed to find other Python deps (e.g. libintl) in Homebrew if it has
nonstandard prefix (e.g. in Apple M1)

* Re-allow to search Homebrew for zlib everywhere

Don't override a user-supplied value

Minor changes to make the docs better.