Remove outdated script simple_rtu_data_store.py

The dev depedencies have been updated to a recent version.

Support for Python 3.3 has been removed.  Python 3.3 is depricated as of
2017-09-29. Travis doesn't support Python 3.3 builds anymore.

On the other side, support for Python 3.7 and Python 3.8 have been
added.

Fixes: #76

The error codes where off by 1.

Fixes: #90

Fix 2 error codes

Instead of declaring the create_from_request_pdu and create_from_response_pdu factory methods as @staticmethod, use @classmethod. Then the class is passed to the method as the cls parameter, which can be used to instantiate an instance, rather than hardcoding the class name.

This allows the function classes to be subclassed without having to reimplement the factory methods just to change the class name.

Also fixes up some comments that were incorrect.

`log.exception` already includes a traceback, so don't add another.

The definition of umodbus.server.route gives default values of None to the arguments, suggesting they can be omitted. However, if you actually provide None, it will fail later when evaluating DataRule.match.

This change makes it possible to omit any rule constraint to match on any value.

For full bytes, the Byte Count field was calculated too high:
    8 // 8 + 1 = 2
while only one byte is needed to represent 8 coils.

Offset the values length before the truncating division to correctly
round to whole bytes.  This is easier than what the standard
formulates:
    N = Quantity of Outputs / 8, if the
    remainder is different of 0 ==> N = N+1

Note that the actual data bytes were already handled with the correct
length.  So in case of 8 coils, uModbus would send a Byte Count of 2,
but only one data byte before the checksum.

The server example had port 502 hard coded. This port is a privileged port.
Trying to use it without permissions could result in a failure of the example.
Port 502 is still the default, but a nice error message is printed to warn the user
about the problem. Also a flag has been introduced to change the port.

To keep the example client working with the example server code a similar
change has been made to the example client.