Docs: oauth2-proxy cookie secret command uses outdated syntax

[clementpsp]

Describe the bug

Recommended oauth2-proxy cookie secret generation command does not follow python3 syntax: https://github.com/search?q=repo%3ASAP%2Fkubeapps%20urlsafe_b64encode&type=code

Ref: https://docs.python.org/3/whatsnew/3.0.html#print-is-a-function

To Reproduce
Steps to reproduce the behavior:

$ python3 -c 'import os,base64; print base64.urlsafe_b64encode(os.urandom(16))'
  File "<string>", line 1
    import os,base64; print base64.urlsafe_b64encode(os.urandom(16))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

Expected behavior

On python2, that command would have produced something like:

$ python -c 'import os,base64; print base64.urlsafe_b64encode(os.urandom(16))'
7jp5YGiuIPeq0Gg0lB4Uxg==

The python3 base64.urlsafe_b64encode also has a few changes, including:

return the encoded bytes.

To produce output equivalent with the python2 version, oauth2-proxy now recommends:

python -c 'import os,base64; print(base64.urlsafe_b64encode(os.urandom(32)).decode())'