Update urllib3 to 1.24 (#393)

This PR updates [urllib3](https://pypi.org/project/urllib3) from **1.23** to **1.24**.



<details>
  <summary>Changelog</summary>
  
  
   ### 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&#39;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)
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/urllib3
  - Changelog: https://pyup.io/changelogs/urllib3/
  - Docs: https://urllib3.readthedocs.io/
</details>

Author: pyup-bot

requirements.txt

@@ -288,9 +288,9 @@ idna==2.7 \
 asn1crypto==0.24.0 \
     --hash=sha256:2f1adbb7546ed199e3c90ef23ec95c5cf3585bac7d11fb7eb562a3fe89c64e87 \
     --hash=sha256:9d5c20441baf0cb60a4ac34cc447c6c189024b6b4c6cd7877034f4965c464e49
-urllib3==1.23 \
-    --hash=sha256:b5725a0bd4ba422ab0e66e89e030c806576753ea3ee08554382c14e685d117b5 \
-    --hash=sha256:a68ac5e15e76e7e5dd2b8f94007233e01effe3e50e8daddf69acfd81cb686baf
+urllib3==1.24 \
+    --hash=sha256:8819bba37a02d143296a4d032373c4dd4aca11f6d4c9973335ca75f9c8475f59 \
+    --hash=sha256:41c3db2fc01e5b907288010dec72f9d0a74e37d6994e6eb56849f59fea2265ae
 pycparser==2.19 \
     --hash=sha256:a988718abfad80b6b157acce7bf130a30876d27603738ac39f140993246b25b3
 packaging==18.0 \