XMODEM protocol implementation

The `xmodem` library provides an implementation of the XMODEM, YMODEM, and ZMODEM protocols for Python. It focuses on a minimalistic implementation while adhering to protocol specifications. All modem classes require `getc` and `putc` callback functions to handle character data for communication. The current version is 0.5.0, with releases occurring periodically to address bugs and introduce enhancements.