Invoke config.py instead of config.pl

git grep -Fl /config.pl | xargs sed -i -e 's!/config\.pl!/config.py!g'

Also:
* Change one comment in include/mbedtls/check_config.h.
* Change PERL to PYTHON in CMakeLists.txt.
diff --git a/tests/scripts/key-exchanges.pl b/tests/scripts/key-exchanges.pl
index 3bf7ae3..be029c7 100755
--- a/tests/scripts/key-exchanges.pl
+++ b/tests/scripts/key-exchanges.pl
@@ -47,10 +47,10 @@
     print "******************************************\n";
 
     # full config with all key exchanges disabled except one
-    system( "scripts/config.pl full" ) and abort "Failed config full\n";
+    system( "scripts/config.py full" ) and abort "Failed config full\n";
     for my $k (@kexes) {
         next if $k eq $kex;
-        system( "scripts/config.pl unset $k" )
+        system( "scripts/config.py unset $k" )
             and abort "Failed to disable $k\n";
     }