Yubico-client

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* Update client to query a single API server instead of multiple ones in
parallel by default.

Previously, we queried 6 Yubico API servers in parallel for high availability
and performance reasons.

Now api.yubico.com is globally distributed and load balanced so there is no
need for us to do that anymore and we can just query a single API server and
let the server handle HA and load-balancing.

Users who run their own internal Yubikey validation servers can still specify
a list of servers by passing ``api_urls`` argument to the client constructor
(same as before).

Contributed by mallensb and nrw505. Part of 31 and 32.

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* Update code to retry HTTP requests for server errors which might work on a
retry (5xx status codes returned by the server). Those errors could simply
indicate a gateway or a proxy error which might work on a retry.

Contributed by Nigel Williams (nrw505) 30
* Pin minimum version for ``requests`` dependency to ``v2.22.0``.
* Update the code so we don't throw an exception if one of the multiple servers
we query has issues.

We query multiple servers in parallel purely for availability reasons and the
consistency / sync part is taken care by the server side. The client returns
immediately as soon as it receives one positive or a negative response.

Contributed by Nigel Williams (nrw505) 29
* Update code so we send ``User-Agent`` header which includes client version
information and platform string with each HTTP request.
* Also test the code with Python 3.8.0 and indicate we also support Python 3.8.

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* Drop support for Python 2.6. 28
* Test the code and verify it works with the following Python versions:
* Python 3.3
* Python 3.4
* Python 3.5
* Python 3.6
* Python 3.7
* PyPy 2
* PyPy 3 28

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* Fix compatibility issue with Python versions >= 3.0 <= 3.3 22
* Pin ``requests`` dependency to the latest version (2.7) 25 27

Contribution by Wouter van Bommel, Vianney Carel.

* Make sure the query parameters are unquoted when parsing them from the
response. 23

Contribution by Tamás Gulácsi.

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* Fix Python 3 compatibility issue. 21

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* To discourage bad practices, remove ``use_https`` argument from the `Yubico`
class constructor all together. Also update ``DEFAULT_API_URLS`` variable to
contain full URLs with a scheme (e.g.
``https://api.yubico.com/wsapi/2.0/verify``).

If a user wants to use a custom non-https URL or URLs, they can still do that
by passing ``api_urls`` argument with custom non-https URLs to the
constructor.

* Replace ``CA_CERTS_BUNDLE_PATH`` module level variable with a
``ca_certs_bundle_path`` argument which can be passed to the Yubico class
constructor.

* Update ``requests`` dependency from ``1.2`` to ``2.2``.