update generate_reqs invoke task for latest pipenv

Author: Kilo59

tasks.py

@@ -72,9 +72,11 @@ def test(ctx):
 @invoke.task
 def generate_reqs(ctx):
     """Generate requirements.txt"""
+    # NOTE: updated for pipenv 2020 release
+    # TODO: make backwards compatible
     reqs = [
         "pipenv lock -r > requirements.txt",
-        "pipenv lock -r --dev > requirements-dev.txt",
+        "pipenv lock -r --dev-only > requirements-dev.txt",
     ]
     [ctx.run(req) for req in reqs]