Change from ffi to ffi-napi since the former seems unsupported

This should also fix installation problem of node-gyp in CI

src/runtime/python/README.md

@@ -3,7 +3,8 @@
 ## Pre-requisites
 
 1. You must have installed the PGF C runtime (see `../c/README.md`)
-2. You will need the system Python development package, e.g.:
+2. You may need to set the environment variable `LD_LIBRARY_PATH=/usr/local/lib`
+3. You will need the system Python development package, e.g.:
   - RedHat: `yum install python-devel`
   - Debian: `apt install python-dev`
 
@@ -25,3 +26,10 @@ pytest
 ```
 
 Requires: `pip install pytest`
+
+
+## Debugging
+
+valgrind --leak-check=full --show-leak-kinds=all pytest 2> valgrind.txt
+
+valgrind --leak-check=full --show-leak-kinds=all -- python -c 'import pgf; s = pgf.readType("A -> B"); print(s)' 2> valgrind.txt