Add type annotations for the SSL module (#1308)

* Add type annotations for the `SSL` module

Instead of relying on the third-party `types-pyOpenSSL` package, provide
our own type annotations. This will ensure `pyOpenSSL` users always have
type annotations which matches their version.

* Replace `py.typed` by an empty file

Author: jlaine

MANIFEST.in

@@ -1,4 +1,4 @@
-include             LICENSE MANIFEST.in *.rst tox.ini .coveragerc
+include             LICENSE MANIFEST.in *.rst tox.ini .coveragerc src/OpenSSL/py.typed
 exclude             codecov.yml .readthedocs.yml mypy.ini
 recursive-include   tests           *.py
 recursive-include   doc             *