python3 compatibility: AttributeError: 'HTTPMessage' object has no attribute 'getheaders'

[Florian Wolff]

Issue exists on xenial in python3-paste 1.7.5.1-6ubuntu3 which throws exception:

   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 1068, in process_request_in_thread
     self.finish_request(request, client_address)
   File "/usr/lib/python3.5/socketserver.py", line 354, in finish_request
     self.RequestHandlerClass(request, client_address, self)
   File "/usr/lib/python3.5/socketserver.py", line 681, in __init__
     self.handle()
   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 442, in handle
     BaseHTTPRequestHandler.handle(self)
   File "/usr/lib/python3.5/http/server.py", line 422, in handle
     self.handle_one_request()
   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 437, in handle_one_request
     self.wsgi_execute()
   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 283, in wsgi_execute
     self.wsgi_setup(environ)
   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 255, in wsgi_setup
     self.wsgi_environ[key] = ','.join(self.headers.getheaders(k))
 AttributeError: 'HTTPMessage' object has no attribute 'getheaders'
 Exception in thread worker 5:
 Traceback (most recent call last):
   File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
     self.run()
   File "/usr/lib/python3.5/threading.py", line 862, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/paste/httpserver.py", line 895, in worker_thread_callback
     sys.exc_clear()
 AttributeError: module 'sys' has no attribute 'exc_clear'

due to changes in python3 (3.5.2-2ubuntu0~16.04.10):
- exc_clear() call is no longer required (ref https://docs.python.org/3/whatsnew/3.0.html#index-22 )
- getheaders was renamed to get (ref https://bugs.launchpad.net/python-keystoneclient/+bug/1267987 )