fix: strip trailing _ from field mask paths (#228)

noxfile.py

diff --git a/noxfile.py b/noxfile.py
index 2560992..2f11137 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -98,9 +98,10 @@
     ]
     pytest_args.extend(session.posargs)
 
-    # Inject AsyncIO content, if version >= 3.6.
+    # Inject AsyncIO content and proto-plus, if version >= 3.6.
+    # proto-plus is needed for a field mask test in test_protobuf_helpers.py
     if _greater_or_equal_than_36(session.python):
-        session.install("asyncmock", "pytest-asyncio")
+        session.install("asyncmock", "pytest-asyncio", "proto-plus")
 
         pytest_args.append("--cov=tests.asyncio")
         pytest_args.append(os.path.join("tests", "asyncio"))