port 2P step 1: vendor CPython 2.7.18 as the mtpython static library

The embedded 40250 script layer needs its own interpreter on five platforms.
2.7 is end-of-life, so nothing can be fetched from the target SDKs and the
source is vendored (official 2.7.18 tarball, trimmed to 25MB; Lib/ stays for
step 3's python27.zip).

cpython-2.7.18/CMakeLists.txt builds one `mtpython` static library from exactly
the 133 objects the reference libpython2.7.a contains, off by default behind
-DMTGODOT_EMBED_PYTHON=ON. The source list and the built-in module table
(config/config.c, 39 entries) are shared; pyconfig.h is a probe result and is
not, so each platform keeps its own under config/<platform>/, regenerated by
tools/py_embed/gen_pyconfig.sh and checked against the shared table.

Three vendor patches, documented in docs/THIRD-PARTY.md: configure/configure.ac
learn arm64 on macOS, and posixmodule.c undefines the process-control calls it
hard-defines past pyconfig.h when building for iOS.

macOS (ctest python.embed: every builtin imports, codecs and pickle work off the
vendored Lib/), Android arm64 and iOS arm64 build. Linux needs a Linux host and
Windows needs a MinGW-vs-MSVC decision; both are noted for step 4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/third_party/cpython-2.7.18/Lib/email/mime/base.py

@@ -0,0 +1,26 @@
+# Copyright (C) 2001-2006 Python Software Foundation
+# Author: Barry Warsaw
+# Contact: email-sig@python.org
+
+"""Base class for MIME specializations."""
+
+__all__ = ['MIMEBase']
+
+from email import message
+
+
+
+class MIMEBase(message.Message):
+    """Base class for MIME specializations."""
+
+    def __init__(self, _maintype, _subtype, **_params):
+        """This constructor adds a Content-Type: and a MIME-Version: header.
+
+        The Content-Type: header is taken from the _maintype and _subtype
+        arguments.  Additional parameters for this header are taken from the
+        keyword arguments.
+        """
+        message.Message.__init__(self)
+        ctype = '%s/%s' % (_maintype, _subtype)
+        self.add_header('Content-Type', ctype, **_params)
+        self['MIME-Version'] = '1.0'