Couple of points:

1. On your last example, which webserver treats 'L' as part of port number? I can't of anything.

2. Can you write a "real application" which is listening to beyond 65535? Which platform would it be?

Current way of handling invalid port like, int('foo') by raising ValueError seems to be a better than returning a None.  A better error message could be desirable, but that does not make it a security issue.

Additionally, for the example of int changing long integer to 'L' appended one would a 2.x case as it is no longer the behavior in 3.x

Also, I would advice to look at getPort function in a C library or a Java library and see what it does. The only difference I see is, they return -1 where Python returns None.

I am changing the request type to an enhancement, because there is not a valid argument to support that it is a security issue.