Bump pyOpenSSL compat range; add dep version strategy note

Author: kislyuk

setup.py

@@ -12,10 +12,12 @@
     description='Python XML Signature library',
     long_description=open('README.rst').read(),
     install_requires=[
+        # Dependencies are restricted by major version range according to semver.
+        # By default, version minimums are set to be compatible with the oldest supported Ubuntu LTS (currently 18.04).
         'lxml >= 4.2.1, < 5',
         'eight >= 0.4.2, < 2',
         'cryptography >= 2.1.4, < 4',
-        'pyOpenSSL >= 17.5.0, < 20',
+        'pyOpenSSL >= 17.5.0, < 21',
         'certifi >= 2018.1.18'
     ],
     extras_require={