Use prepare_metadata with Pip 19 instead of run_egg_info

Newer Pip [1] doesn't fill the requirement metadata anymore with the
"run_egg_info" method, but a new method named "prepare_metadata" should
be used instead.  Make the "name_from_ireq" helper function to call the
new method when it's available.

[1] pypa/pip#5743

prequ/utils.py

@@ -45,8 +45,11 @@ def name_from_ireq(ireq):
     """
     if not ireq.req:
         ireq.source_dir = os.path.abspath(ireq.source_dir)
-        ireq.run_egg_info()
-        assert ireq.req, "run_egg_info should fill req: {!r}".format(ireq)
+        if hasattr(ireq, 'prepare_metadata'):
+            ireq.prepare_metadata()
+        else:
+            ireq.run_egg_info()
+        assert ireq.req, "prepare_metadata should fill req: {!r}".format(ireq)
     return name_from_req(ireq.req)