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/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index ede0704..999fc52 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -45,7 +45,7 @@
 #endif
 
 /* Fix the config here. Not convenient to put an #ifdef _WIN32 in config.h as
- * it would confuse config.pl. */
+ * it would confuse config.py. */
 #if !defined(MBEDTLS_PLATFORM_SNPRINTF_ALT) && \
     !defined(MBEDTLS_PLATFORM_SNPRINTF_MACRO)
 #define MBEDTLS_PLATFORM_SNPRINTF_ALT