Tear out remainders of Python 2 support

[C0rn3j]

trafficstars

Remove bits of Python 2 support, people desiring to use an EOL version of Python with this library can choose the currently released version.

[BYVoid]

What's the problem with the support of Python 2?

[C0rn3j]

Inability to use modern language features.

Nobody is running a dead CentOS 5 box with Python 2 and using the 2025 version of OpenCC, no point in crippling the project for nonexistent users that could use the previous versions anyway.

https://devguide.python.org/versions/