Improve the clarity of error messages for version incompatibility

[comrumino]

There should be a unique exception which is thrown when using incompatible versions. Rather than using an exception hierarchy, once Python 2 support is dropped, RpycVersionError should suffice. So, support for this should be done in tandem with dropping Python 2 support.

Environment: RPyC versions

Original issue: #330
Client

• RPyC 4.1.0
• Python 3

Server

• RPyC 4.0.2
• Python 3

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/rpyc/core/protocol.py", line 329, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
  File "/usr/lib/python3.7/site-packages/rpyc/core/protocol.py", line 594, in _handle_inspect
    return tuple(netref.inspect_methods(self._local_objects[oid]))
  File "/usr/lib/python3.7/site-packages/rpyc/lib/colls.py", line 86, in __getitem__
    return self._dict[key][0]
KeyError: (7696561775896, 'SlaveService', 'rpyc.core.service')

Environment: Python interpreter 2-3

Original issue: #327
Client

• RPyC 4.1.0
• Python 2

Server

• RPyC 4.1.0
• Python 3

Traceback (most recent call last):
  File "./matlabClient.py", line 17, in <module>
    r = c.root.performMatlab("gcd", *l)
  File "/home/lafayette/.local/lib/python2.7/site-packages/rpyc/core/netref.py", line 199, in __call__
    return syncreq(_self, consts.HANDLE_CALL, args, kwargs)
  File "/home/lafayette/.local/lib/python2.7/site-packages/rpyc/core/netref.py", line 75, in syncreq
    return conn.sync_request(handler, proxy, *args)
  File "/home/lafayette/.local/lib/python2.7/site-packages/rpyc/core/protocol.py", line 471, in sync_request
    return self.async_request(handler, *args, timeout=timeout).value
  File "/home/lafayette/.local/lib/python2.7/site-packages/rpyc/core/async_.py", line 97, in value
    raise self._obj
rpyc.core.vinegar/builtins.TypeError: must be str, not bytes

========= Remote Traceback (1) =========
Traceback (most recent call last):
  File "/home/lafayette/.local/lib/python3.6/site-packages/rpyc/core/protocol.py", line 329, in _dispatch_request
    res = self._HANDLERS[handler](self, *args)
  File "/home/lafayette/.local/lib/python3.6/site-packages/rpyc/core/protocol.py", line 590, in _handle_call
    return obj(*args, **dict(kwargs))
  File "matlabService.py", line 162, in exposed_performMatlab
    return performMatlab(theMatlabFunc, *args, **kwargs)
  File "matlabService.py", line 121, in performMatlab
    " " + repr(args) + " " + repr(kwargs))
TypeError: must be str, not bytes