macos: set MACOSX_DEPLOYMENT_TARGET to 10.9

Mavericks was released in October 2013. Hopefully that is old enough.
10.9 is also the minimum version that Firefox requires and Firefox's
policy is generally grounded in real world usage.

Author: indygreg

cpython-macos/build.py

@@ -32,7 +32,7 @@
 BUILD = ROOT / 'build'
 SUPPORT = ROOT / 'cpython-macos'
 
-MACOSX_DEPLOYMENT_TARGET = '10.14'
+MACOSX_DEPLOYMENT_TARGET = '10.9'
 
 DISABLED_STATIC_MODULES = {
     # We don't support GDBM because it is GPL v3.