1160794 - update python-requests to 2.4.3

python-requests 2.2.1 appears to have a bug that prevents re-using the same
session when downloading SSL content via proxy. The code below will work on
2.4.3 but not 2.2.1.

```python
import requests
from urlparse import urljoin

proxy_url = "http://<proxy_hostname>:<port>"
proxies = {"http": proxy_url, "https": proxy_url}
base_url = "https://repos.fedorapeople.org/repos/pulp/pulp/demo_repos/pulp_unittest/"

s = requests.Session()
print "Attempting to retrieve two files from %s using the same session..." % base_url
print s.get(urljoin(base_url, "comps.xml"), proxies=proxies)
print s.get(urljoin(base_url, "updateinfo.xml"), proxies=proxies)
```

python-requests 2.4.3 allows Pulp to download content in these cases. Note that
not all repos with SSL enabled are affected by this.
Merge pull request #1290 from beav/requests-243

1160794 - update python-requests to 2.4.3