Add rfc2217 support

[Andre0512]

Solution to #46 - support for rfc2217/ser2net connections.

The polling approach from windows worked so far, but I needed the self attribute to distinguish the platform/protocol, so I merged the two solutions.

[joncar]

The same problem happens for other non-file-based serial implementations, such as anything custom registered using serial.protocol_handler_packages.append("test_handlers"). I wonder if a more universal approach would be to attempt the fileno() call during initialization: if successful go down the add_writer/add_reader path and if it throws an exception go down the polling path.

[joncar]

Yup, that does seem to work great. I tweaked your change to make my test that uses a custom URL handler work: joncar@4903067. (I didn't test it on Windows or with rfc2217 though!)

[Andre0512]

I like your solution, that's a much better approach. I added your commit to my pr, hope that's okay for you. I tested with rfc2217 and it works well.

[bluespider42]

Any chance this will be merged?