Always use the RequestsTransport to access pypi

[filmor]

This ensures that proxies from the environment are used even if they are not defined in the .condarc.

Fixes #550.

[msarahan]

I'm not familiar with this code, nor really familiar with proxies. Can you describe what problem led you to this change? How does this affect proxy settings in condarc? What would it take to add a test for the functionality you desire?

[filmor]

Check issue #550. conda skeleton pypi (implemented in pypi.py) has code to use proxy servers (the mentioned RequestsTransport) correctly through the CondaSession object. However, this code is only invoked if the .condarc file contains the proxies entry, in particular, it skips the (otherwise working) $HTTP_PROXY etc. environment variables.

Regarding tests, I guess for those you would need a proxied environment on the CI servers, not configure the proxy server in .condarc but instead only in the environment and run conda skeleton pypi some-package there.

[msarahan]

@filmor thanks for this. I don't fully grok all of this, but it looks like a good improvement to me. I'm going to merge it. Is it OK to ping you if I run across issues/concerns?