Add pyflakes to linting setup.

[sallner]

In the test to the secrets an undefined variable secrets occurred.

batou/src/batou/secrets/tests/test_secrets.py

Lines 59 to 65 in 6380387

	def test_decrypt_missing_key(monkeypatch):
	monkeypatch.setitem(os.environ, "GNUPGHOME", '/tmp')
	with pytest.raises(subprocess.CalledProcessError):
	EncryptedConfigFile(encrypted_file)
	f = secrets.__enter__()
	f.read()

I would propose to add pyflakes to the linting chain, to ensure those kind of errors do not happen.

A run over the current codebase resulted in the following output.

[batou]$pyflakes src/
src/batou/deploy.py:9:1 'batou.ConfigurationError' imported but unused
src/batou/deploy.py:9:1 'batou.DeploymentError' imported but unused
src/batou/deploy.py:9:1 'batou.FileLockedError' imported but unused
src/batou/deploy.py:13:1 '.environment.MissingEnvironment' imported but unused
src/batou/remote_core.py:392:47 undefined name 'e'
src/batou/conftest.py:7:12 local variable 'FIXTURE' is assigned to but never used
src/batou/utils.py:111:25 '...'.format(...) has unused arguments at position(s): 0, 1
src/batou/fixtures.py:2:1 'subprocess' imported but unused
src/batou/secrets/encryption.py:7:1 'shlex' imported but unused
src/batou/secrets/edit.py:2:1 'glob' imported but unused
src/batou/secrets/tests/test_secrets.py:64:13 undefined name 'secrets'
src/batou/tests/test_remote.py:40:5 local variable 'd' is assigned to but never used
src/batou/tests/test_remote_core.py:111:70 local variable 'f' is assigned to but never used
src/batou/tests/test_utils.py:60:39 local variable 'f' is assigned to but never used
``

[sallner]

When working on #184 I actually run into the problem with the test mentioned above. I actually encountered a NameError in this test now. I would like to prioritize this issue higher.

[ctheune]

I started working on this and I'm running into some interesting things about the linter setup: depending on the way we call the (now) three main tools (flake8, isort, yapf) we seem to get different results:

• from my editor
• from precommit
• from precommit when run through tox

In general I think we need to try to convince all tools to use the settings in setup.cfg.