Support disabling C extensions via IMMUTABLES_EXT=0

Support using IMMUTABLES_EXT environment variable to control whether
the C extension is built explicitly.  When set to a value other than 1,
the extension built is disabled.  This is helpful e.g. for future Python
versions where the extension does not work (this is affecting 3.13 right
now, but having an explicit option is more future-proof).

setup.py

@@ -27,7 +27,8 @@
             'unable to read the version from immutables/_version.py')
 
 
-if platform.python_implementation() == 'CPython':
+if (platform.python_implementation() == 'CPython' and
+        os.environ.get('IMMUTABLES_EXT', '1') == '1'):
     if os.environ.get("DEBUG_IMMUTABLES") == '1':
         define_macros = []
         undef_macros = ['NDEBUG']