Fix bug in Python's THttpClient proxy handling

[pspeter]

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Previously, creating an instance of this class would throw whenever the http(s)_proxy environment variables were set, since it tried concatenating a string with a bytes array.

[pspeter]

@fishy the return type of the function should now always be str in Python 2 and 3, so it is consistent to the current behavior in Python2 while also fixing the function in Python 3.

[fishy]

This is the correct type, yes, but it's not the correct string:

$ python3
Python 3.10.4 (main, Mar 24 2022, 13:07:27) [GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import base64
>>> str(base64.b64encode(b'foo'))
"b'Zm9v'"

[pspeter]

You're right, ensure_str it is.

Python 3:

>>> cr = b"abc"
>>> "Basic " + six.ensure_str(cr)
'Basic abc'

Python 2:

>>> cr = "abc"
>>> "Basic " + six.ensure_str(cr)
'Basic abc'

Thanks for your sharp eye! :)