Update urllib3 to 1.24

[pyup-bot]

This PR updates urllib3 from 1.23 to 1.24.

Changelog

### 1.24 ``` ----------------- * Allow key_server_hostname to be specified when initializing a PoolManager to allow custom SNI to be overridden. (Pull 1449) * Test against Python 3.7 on AppVeyor. (Pull 1453) * Early-out ipv6 checks when running on App Engine. (Pull 1450) * Change ambiguous description of backoff_factor (Pull 1436) * Add ability to handle multiple Content-Encodings (Issue 1441 and Pull 1442) * Skip DNS names that can't be idna-decoded when using pyOpenSSL (Issue 1405). * Add a server_hostname parameter to HTTPSConnection which allows for overriding the SNI hostname sent in the handshake. (Pull 1397) * Drop support for EOL Python 2.6 (Pull 1429 and Pull 1430) * Fixed bug where responses with header Content-Type: message/* erroneously raised HeaderParsingError, resulting in a warning being logged. (Pull 1439) * Move urllib3 to src/urllib3 (Pull 1409) ```

Links

- PyPI: https://pypi.org/project/urllib3 - Changelog: https://pyup.io/changelogs/urllib3/ - Docs: https://urllib3.readthedocs.io/

[coveralls]

Pull Request Test Coverage Report for Build 735

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.956%

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Totals
Change from base Build 553:	0.0%
Covered Lines:	4408
Relevant Lines:	4500

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💛 - Coveralls

[pyup-bot]

Closing this in favor of #134