fix(python): handling support for python 3.6.4

[ehdsouza]

Got the following two errors in python 3.6.4:
*

Traceback (most recent call last):
  File "examples/speech_to_text_v1.py", line 57, in <module>
    audio=audio_file, recognize_callback=mycallback)
  File "/Users/erikadsouza/anaconda3/envs/testpypi/lib/python3.6/site-packages/watson_developer_cloud/speech_to_text_v1.py", line 245, in recognize_with_websocket
    self.username + ':' + self.password)
  File "/Users/erikadsouza/anaconda3/envs/testpypi/lib/python3.6/base64.py", line 58, in b64encode
    encoded = binascii.b2a_base64(s, newline=False)
TypeError: a bytes-like object is required, not 'str'

• urlencode support

[germanattanasio]

🤔

[ehdsouza]

This is for the error in python 3.6.4:

24, in onMessage
    self.sendMessage(self.build_close_message())
  File "/Users/erikadsouza/anaconda3/envs/python-3.6.4/lib/python3.6/site-packages/autobahn/websocket/protocol.py", line 2164, in sendMessage
    assert(type(payload) == bytes)
builtins.AssertionError:

[codecov-io]

Codecov Report

Merging #399 into develop will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           develop    #399   +/-   ##
=======================================
  Coverage        0%      0%           
=======================================
  Files           21      21           
  Lines        10614   10618    +4     
=======================================
- Misses       10614   10618    +4

Impacted Files	Coverage Δ
watson_developer_cloud/speech_to_text_v1.py	0% <0%> (ø)	⬆️
...oud/websocket/speech_to_text_websocket_listener.py	0% <0%> (ø)	⬆️

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[germanattanasio]

This should be os.getenv('DISCOVERY_USERNAME')