Merge pull request #189 from SparkPost/vulnerable_dep

requests 2.5.1 contains two known vulnerabilities

Author: orval

setup.py

@@ -14,7 +14,7 @@
     license='Apache 2.0',
     description='SparkPost Python API client',
     long_description=readme,
-    install_requires=['requests>=2.5.1'],
+    install_requires=['requests>=2.20.1'],
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',