Update requests to 2.19.0

[pyup-bot]

This PR updates requests from 2.18.4 to 2.19.0.

Changelog

2.19.0

++++++++++++++++++

**Improvements**

- Warn user about possible slowdown when using cryptography version < 1.3.4
- Check for invalid host in proxy URL, before forwarding request to adapter.
- Fragments are now properly maintained across redirects. (RFC7231 7.1.2)
- Removed use of cgi module to expedite library load time.
- Added support for SHA-256 and SHA-512 digest auth algorithms.
- Minor performance improvement to ``Request.content``.
- Migrate to using collections.abc for 3.7 compatibility.

**Bugfixes**

- Parsing empty ``Link`` headers with ``parse_header_links()`` no longer return one bogus entry.
- Fixed issue where loading the default certificate bundle from a zip archive
would raise an ``IOError``.
- Fixed issue with unexpected ``ImportError`` on windows system which do not support ``winreg`` module.
- DNS resolution in proxy bypass no longer includes the username and password in
the request. This also fixes the issue of DNS queries failing on macOS.
- Properly normalize adapter prefixes for url comparison.
- Passing ``None`` as a file pointer to the ``files`` param no longer raises an exception.
- Calling ``copy`` on a ``RequestsCookieJar`` will now preserve the cookie policy correctly.

**Dependencies**

- We now support idna v2.7.
- We now support urllib3 v1.23.

Links

• PyPI: https://pypi.org/project/requests
• Changelog: https://pyup.io/changelogs/requests/
• Homepage: http://python-requests.org

Resolves #188