[Bug fix] Allow redirects for proxy

[GrantAnt]

In the current implementation, proxies are offered but not usable.
In case requests library has proxy options set, allow_redirect needs to be set to true.

[google-cla]

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[GrantAnt]

@lzchen Build Job for py39 failed. It seems not be connected to my code changes and last week the build was working on master.
Just restart build tool?

[lzchen]

@GrantAnt

Looks like build is failing for lint.

[GrantAnt]

@lzchen which linter is executed? Can I run it locally?

[lzchen]

@GrantAnt

Looks like it's flake8. You can see the lint steps here: https://github.com/census-instrumentation/opencensus-python/blob/master/tox.ini#L68

[GrantAnt]

@lzchen How could I be so blind. Thank you so much :) Whitespaces are resolved now