Bump license date and remove outdated section in readme

Author: lgrahl

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Lennart Grahl
+Copyright (c) 2021 Lennart Grahl
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.rst

@@ -97,27 +97,6 @@ command:
 
     $ threema-gateway version
 
-Troubleshooting
-***************
-
-**Invalid environment marker: python_version<="3.4"**
-
-If you're getting the following error when installing the library::
-
-    Unpacking /tmp/threema-msgapi-sdk-python
-      Running setup.py (path:/tmp/pip-5b1d7h0w-build/setup.py) egg_info for package from file:///tmp/threema-msgapi-sdk-python
-        error in threema.gateway setup command: Invalid environment marker: python_version<="3.4"
-        Complete output from command python setup.py egg_info:
-        error in threema.gateway setup command: Invalid environment marker: python_version<="3.4"
-
-... then you have an old version of setuptools installed. If you use a venv (as
-described in the "Prerequisites" section), enter it (using ``source
-venv/bin/activate``) and then upgrade setuptools::
-
-    $ pip install -U setuptools
-
-Now installing the ``threema.gateway`` module should work.
-
 Contributing
 ************