[OASIS-7552] fix: update requests library dependency (#330)

* feat: reak down requests[security] into separate modules

* try compatible versions of modules

* try compatible versions of modules

* try compatible versions of modules

* try compatible versions of modules

* Update README with code credits

Author: Mat001

README.md

@@ -224,3 +224,10 @@ would be:
 ### Contributing
 
 Please see [CONTRIBUTING](https://github.com/optimizely/python-sdk/blob/master/CONTRIBUTING.md).
+
+### Additional Code
+This software incorporates code from the following open source repos:  
+requests (Apache-2.0 License: https://github.com/psf/requests/blob/master/LICENSE)  
+pyOpenSSL (Apache-2.0 License https://github.com/pyca/pyopenssl/blob/main/LICENSE)  
+cryptography (Apache-2.0 https://github.com/pyca/cryptography/blob/main/LICENSE.APACHE)  
+idna (BSD 3-Clause License https://github.com/kjd/idna/blob/master/LICENSE.md) 

requirements/core.txt

@@ -1,4 +1,7 @@
 jsonschema==3.2.0
 pyrsistent==0.14.0
 mmh3==2.5.1
-requests[security]>=2.9.1
+requests>=2.21
+pyOpenSSL>=19.1.0
+cryptography>=2.8.0
+idna>=2.10