Add a flake8 run to Tox config, fix its complaints

Author: encukou

pello/__init__.py

@@ -1 +1,3 @@
 from pello.pello_greeting import greeting
+
+__all__ = ['greeting']

pello/__main__.py

@@ -1,4 +1,3 @@
 from pello import pello_greeting
 
 pello_greeting.greeting()
-

pello/pello_greeting.py

@@ -5,4 +5,3 @@ def greeting():
         print(term.green + term.bold + "Hello World!" + term.normal)
     except ImportError:
         print("Hello World!")
-

pyproject.toml

@@ -6,9 +6,13 @@ build-backend = "setuptools.build_meta"
 [tool.tox]
 legacy_tox_ini = """
 [tox]
-envlist = py36, py37, py38, py39
+envlist = py36, py37, py38, py39, lint
 
 [testenv]
 deps = pytest >= 3.0.0
 commands = pytest
+
+[testenv:lint]
+deps = flake8
+commands = flake8 pello
 """