cleanup & test fix

[ThunderEX]

This PR include:

1. remove dependency on copy module which is unnecessary, that also allow pyaes run on micropython.
2. fix imports. for tests, correct path of import, use recommended absolute import to replace relative import, rename StringIO to BytesIO to fit real variable type.
3. remove ugly workaround for py2/3, replace with bytes/bytearray.
4. don't mask MSB, increase speed a little bit

[ricmoo]

Awesome work! Thank you.

I haven't dealt much with Python 3, but will research these changes next week and verify the test cases, and if all is good merge. :)

[ThunderEX]

@ricmoo I mistakenly break down python2 in last commit and now fix it up. pls have a look~

[ricmoo]

Thanks! I'll look at this next week, as this week is pretty busy. I have a lot of house cleaning to catch up on for my various GitHub projects...