split off dependencies for optional installation (#546)

* add inheritance diagrams to all printers and exceptions
* split off printer implementations into separate files
* add wrapper that thros RuntimeError if not importable
* add dependency check for lp
* add dependency check for pyserial
* added check for usability
* import Win32Raw
* include WIn32Raw in documentation
* enable all extras on tox
* update github workflow

src/escpos/printer/dummy.py

@@ -0,0 +1,70 @@
+#!/usr/bin/python
+#  -*- coding: utf-8 -*-
+"""This module contains the implementation of the CupsPrinter printer driver.
+
+:author: python-escpos developers
+:organization: `python-escpos <https://github.com/python-escpos>`_
+:copyright: Copyright (c) 2012-2023 Bashlinux and python-escpos
+:license: MIT
+"""
+
+from ..escpos import Escpos
+
+
+def is_usable() -> bool:
+    """Indicate whether this component can be used due to dependencies."""
+    return True
+
+
+class Dummy(Escpos):
+    """Dummy printer.
+
+    This class is used for saving commands to a variable, for use in situations where
+    there is no need to send commands to an actual printer. This includes
+    generating print jobs for later use, or testing output.
+
+    inheritance:
+
+    .. inheritance-diagram:: escpos.printer.Dummy
+        :parts: 1
+
+    """
+
+    @staticmethod
+    def is_usable() -> bool:
+        """Indicate whether this printer class is usable.
+
+        Will return True if dependencies are available.
+        Will return False if not.
+        """
+        return is_usable()
+
+    def __init__(self, *args, **kwargs):
+        """Init with empty output list."""
+        Escpos.__init__(self, *args, **kwargs)
+        self._output_list = []
+
+    def _raw(self, msg):
+        """Print any command sent in raw format.
+
+        :param msg: arbitrary code to be printed
+        :type msg: bytes
+        """
+        self._output_list.append(msg)
+
+    @property
+    def output(self):
+        """Get the data that was sent to this printer."""
+        return b"".join(self._output_list)
+
+    def clear(self):
+        """Clear the buffer of the printer.
+
+        This method can be called if you send the contents to a physical printer
+        and want to use the Dummy printer for new output.
+        """
+        del self._output_list[:]
+
+    def close(self):
+        """Close not implemented for Dummy printer."""
+        pass