portage/README: Update minimum Python version

* Per commit e59ec19 Py 2.7 has been
  dropped and py3.6 is the minimum supported version

Bug: https://bugs.gentoo.org/731114
Signed-off-by: Aaron Bauman <bman@gentoo.org>
Signed-off-by: Zac Medico <zmedico@gentoo.org>

README

@@ -10,7 +10,7 @@ other package managers.
 Dependencies
 ============
 
-Python and Bash should be the only hard dependencies. Python 2.7 is the
+Python and Bash should be the only hard dependencies. Python 3.6 is the
 minimum supported version.
 
 Native Extensions