Support for Python3

[suyash-chavan]

L2 tester works fine for Pyhon2.7 but there is some problem while building l2tester for Python 3.8. Packets are being not send as expected.

[gringolito]

Please take a look at PR #18 and if possible try to run some tests on your application. Feel free to contribute to this PR and point any issues related to the Python3 support that you possibly find.