Use system's CA certificate store

--- a/certifi/core.py
+++ b/certifi/core.py
@@ -10,6 +10,4 @@ import os


 def where():
-    f = os.path.dirname(__file__)
-
-    return os.path.join(f, 'cacert.pem')
+    return "@GENTOO_PORTAGE_EPREFIX@/etc/ssl/certs/ca-certificates.crt"