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/basic-build-test.sh b/tests/scripts/basic-build-test.sh
index 54ca934..7ed0372 100755
--- a/tests/scripts/basic-build-test.sh
+++ b/tests/scripts/basic-build-test.sh
@@ -46,8 +46,8 @@
 export LDFLAGS=' --coverage'
 make clean
 cp "$CONFIG_H" "$CONFIG_BAK"
-scripts/config.pl full
-scripts/config.pl unset MBEDTLS_MEMORY_BACKTRACE
+scripts/config.py full
+scripts/config.py unset MBEDTLS_MEMORY_BACKTRACE
 make -j