Fix Python3 get_python_exec_prefix on Fedora 27

On Fedora 27, the default Pytho 3 install prefix changed from /usr
to /usr/local:
https://fedoraproject.org/wiki/Changes/Making_sudo_pip_safe

Closes-Bug: #1741901
Change-Id: Id40620efdf173189df053b5d380a801092933f83
diff --git a/inc/python b/inc/python
index 2e4eff0..e074ea4 100644
--- a/inc/python
+++ b/inc/python
@@ -49,7 +49,11 @@
     fi
     $xtrace
 
-    if is_fedora || is_suse; then
+    if python3_enabled && [ "$os_VENDOR" = "Fedora" -a $os_RELEASE -gt 26 ]; then
+        # Default Python 3 install prefix changed to /usr/local in Fedora 27:
+        # https://fedoraproject.org/wiki/Changes/Making_sudo_pip_safe
+        echo "/usr/local/bin"
+    elif is_fedora || is_suse; then
         echo "/usr/bin"
     else
         echo "/usr/local/bin"