Now you have something to show the matplotlib folks - although they're not likely to get excited about leaking 40 bytes.

There is nothing Python can do about this.  matplotlib is responsible for free'ing the memory matplotlib allocates, just as Python is responsible for free'ing the memory Python allocates.  If _you_ wrote a C extension module, and did

char *p = (char *)malloc(40);

and never did a free(p) later, it would be the same kind of thing.