Rename main_util_fixtures.py to conftest.py to allow fixture auto discovery

Also fix new PEP-008 issues.

cibuildwheel/__main__.py

@@ -12,7 +12,10 @@ from cibuildwheel.environment import (
     EnvironmentParseError,
     parse_environment,
 )
-from cibuildwheel.util import BuildSelector, Unbuffered
+from cibuildwheel.util import (
+    BuildSelector,
+    Unbuffered
+)
 
 
 def get_option_from_environment(option_name, platform=None, default=None):

cibuildwheel/bashlex_eval.py

@@ -1,6 +1,5 @@
 import shlex
 import subprocess
-import sys
 from collections import namedtuple
 
 import bashlex
@@ -67,6 +66,5 @@ def evaluate_command_node(node, context):
     return subprocess.check_output(shlex.split(command), env=context.environment, universal_newlines=True)
 
 
-
 def evaluate_parameter_node(node, context):
     return context.environment.get(node.value, '')