Add pyflakes check (#3)

Reviewed-on: https://git.deguo.duckdns.org/pavel/argclass/pulls/3
Co-authored-by: Pavel Lutskov <pavel.lutskov@gmail.com>
Co-committed-by: Pavel Lutskov <pavel.lutskov@gmail.com>

.drone.yml

@@ -9,12 +9,19 @@ steps:
       - black --check --diff --quiet .
     depends_on: []
 
+  - name: pyflakes
+    image: alpine/flake8:3.9.2
+    commands:
+      - pyflakes .
+    depends_on: []
+
   - name: test-python-3.8
     image: python:3.8-slim
     commands:
       - python test.py
     depends_on:
       - black
+      - pyflakes
 
   - name: test-python-3.9
     image: python:3.9-slim
@@ -22,3 +29,4 @@ steps:
       - python test.py
     depends_on:
       - black
+      - pyflakes

test.py

@@ -1,4 +1,3 @@
-from argparse import ArgumentError, ArgumentTypeError
 import unittest
 from typing import List