Re-enable HTTPS tests for download timeouts

[MRG+1] declare Python 3 support in setup.py

[MRG+1] Extract a function to build CONNECT request; add tests for it. 

Build universal wheels

[MRG+1] Update release notes about change of default S3 ACL policy to "private"

[MRG+1] Refactored SpiderLoader class constructor for easier subclassing

[MRG+1] Document DOWNLOADER_* settings for HTTP/1.0 and TLS

[MRG+1] Use best practices for TLS connections when using Twisted>=14.0

[MRG+1] Change default S3 ACL to "private" and allow customization via settings

Fix documentation for S3_STORE_ACL (now settings.FILES_STORE_S3_ACL) settings: it has nothing to do with feed exporters.

[MRG+1] Add "Content-Length: 0" for body-less HTTP/1.1 POST requests

Revert "Cleanup http11 tunneling connection after #1678"

[MRG+1] Better tracebacks

Fix SkipTest() message for botocore import test

It prints the same as passing `str(e)`

GH-823 was fixed only for HTTP/1.0 (in GH-1089)

Revert test server certificate change

New DOWNLOADER_CLIENT_TLS_METHOD setting to configure TLS method

Fix typos in docstrings