Updates to Python for macOS (#16449)

Co-authored-by: Elizabeth Fischer <elizabeth.fischer@alaska.edu>

var/spack/repos/builtin/packages/python/package.py

@@ -84,8 +84,9 @@ class Python(AutotoolsPackage):
     )
 
     # --enable-shared is known to cause problems for some users on macOS
+    # This is a problem for Python 2.7 only, not Python3
     # See http://bugs.python.org/issue29846
-    variant('shared', default=sys.platform != 'darwin',
+    variant('shared', default=True,
             description='Enable shared libraries')
     # From https://docs.python.org/2/c-api/unicode.html: Python's default
     # builds use a 16-bit type for Py_UNICODE and store Unicode values