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/list-symbols.sh b/tests/scripts/list-symbols.sh
index 6ecc199..1c348a7 100755
--- a/tests/scripts/list-symbols.sh
+++ b/tests/scripts/list-symbols.sh
@@ -13,7 +13,7 @@
 fi
 
 cp include/mbedtls/config.h include/mbedtls/config.h.bak
-scripts/config.pl full
+scripts/config.py full
 make clean
 make_ret=
 CFLAGS=-fno-asynchronous-unwind-tables make lib \