Dynamic scram example

[meejah]

Upgrades the SCRAM example to use dynamic-scram (this is on top of #1024 so merging that first should make this just one commit)

See also crossbario/crossbar#1351

Only merge after the above Crossbar features has been released!

[codecov-io]

Codecov Report

Merging #1025 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1025   +/-   ##
=======================================
  Coverage   61.27%   61.27%           
=======================================
  Files          74       74           
  Lines       13141    13141           
  Branches     2091     2091           
=======================================
  Hits         8052     8052           
  Misses       4386     4386           
  Partials      703      703

Impacted Files	Coverage Δ
autobahn/wamp/test/test_auth.py	100% <ø> (ø)	⬆️
autobahn/wamp/auth.py	64.2% <100%> (ø)	⬆️

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