tests: test that Python documentation renders

A couple of times, I've broken help(drgn) by formatting a function
signature in a way that the inspect module doesn't understand (namely,
it crashes on Enum default arguments). Let's add a simple test that the
documentation at least renders.

tests/test_docs.py

@@ -0,0 +1,9 @@
+import pydoc
+import unittest
+
+import drgn
+
+
+class TestDocs(unittest.TestCase):
+    def test_render(self):
+        pydoc.render_doc(drgn)