setup: Specify some known minimal version requirements

So tests ran on an older system fail with a clean error.

setup.py

@@ -46,8 +46,8 @@ setup(
   packages=find_packages(),
   install_requires=[
     'cryptography', # everything x509 except...
-    'pyOpenSSL', # ...certificate chain validation
-    'pem', # Parse PEM files
+    'pyOpenSSL>=17.1.0', # ...certificate chain validation
+    'pem>=17.1.0', # Parse PEM files
   ],
   tests_require=[
     'coverage',