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

Also fix new PEP-008 issues.

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, '')