build(deps): drop six from requirements

It is only used in a single place to check for Python 2, which can
easily be inlined.

django_migration_linter/migration_linter.py

@@ -5,6 +5,7 @@ import inspect
 import logging
 import os
 import re
+import sys
 from enum import Enum, unique
 from subprocess import PIPE, Popen
 
@@ -12,7 +13,6 @@ from django.conf import settings
 from django.core.management import call_command
 from django.db import DEFAULT_DB_ALIAS, ProgrammingError, connections
 from django.db.migrations import RunPython, RunSQL
-from six import PY2
 
 from .cache import Cache
 from .constants import (
@@ -26,6 +26,8 @@ from .utils import clean_bytes_to_str, get_migration_abspath, split_migration_pa
 
 logger = logging.getLogger("django_migration_linter")
 
+PY2 = sys.version_info[0] == 2
+
 
 @unique
 class MessageType(Enum):