Client sends two `Host` headers about duo_client_python

[AaronAtDuo]

@gkelly Can you confirm you are using Python 3? I can replicate this issue with Python 3.8.5 but not with 2.7.18. On Python 2 I get only one Host header. If possible, could you see if you get the same result for Python 2?

[gkelly]

Yes, I am using Python 3. I just tested this in Python 2.7.14 and did not see the same double-Host-header behavior. However, I also tested #145 in Python 2.7.14 and it does not remove the lone Host header. That is, the request looks correct with the patch applied:

POST /auth/v2/auth HTTP/1.1\r
Host: localhost\r
Accept-Encoding: identity\r
Content-Length: 22\r
Date: Wed, 02 Jun 2021 19:15:49 -0000\r
Content-type: application/x-www-form-urlencoded\r
Authorization: Basic <removed>\r
User-Agent: Duo API Python/4.3.1\r
\r
async=1&factor=testing

[AaronAtDuo]

I agree. So it seems the Py2 behavior of HTTPConnection is:
"Add a Host header if it's not already there"
Where Py3 is
"Add a Host header"

[AaronAtDuo]

Merged! We'll try to get a release done ASAP.

Thanks for the contribution and thanks for using Duo.