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

Also fix new PEP-008 issues.

unit_test/main_tests/conftest.py

@@ -1,10 +1,15 @@
-import pytest
-
-import sys
 import os
 import subprocess
+import sys
 
-from cibuildwheel import linux, macos, windows, util
+import pytest
+
+from cibuildwheel import (
+    linux,
+    macos,
+    util,
+    windows,
+)
 
 
 class ArgsInterceptor:
@@ -15,6 +20,7 @@ class ArgsInterceptor:
 
 MOCK_PROJECT_DIR = 'some_project_dir'
 
+
 @pytest.fixture(autouse=True)
 def mock_protection(monkeypatch):
     '''
@@ -31,13 +37,14 @@ def mock_protection(monkeypatch):
     monkeypatch.setattr(linux, 'build', fail_on_call)
     monkeypatch.setattr(macos, 'build', fail_on_call)
 
+
 @pytest.fixture(autouse=True)
 def fake_project_dir(monkeypatch):
     '''
     Monkey-patch enough for the main() function to run
     '''
-
     real_os_path_exists = os.path.exists
+
     def mock_os_path_exists(path):
         if path == os.path.join(MOCK_PROJECT_DIR, 'setup.py'):
             return True

unit_test/main_tests/main_options_test.py

@@ -1,14 +1,11 @@
-import pytest
-
 import sys
 
+import pytest
+
 from cibuildwheel.__main__ import main
 from cibuildwheel.environment import ParsedEnvironment
 from cibuildwheel.util import BuildSelector
 
-from main_util_fixtures import mock_protection, fake_project_dir, platform, intercepted_build_args
-
-
 
 # CIBW_PLATFORM is tested in main_platform_test.py
 
@@ -93,6 +90,7 @@ def get_default_repair_command(platform):
     else:
         raise ValueError('Unknown platform', platform)
 
+
 @pytest.mark.parametrize('repair_command', [None, 'repair', 'repair -w {dest_dir} {wheel}'])
 @pytest.mark.parametrize('platform_specific', [False, True])
 def test_repair_command(repair_command, platform_specific, platform, intercepted_build_args, monkeypatch):
@@ -225,5 +223,3 @@ def test_build_selector_migrations(intercepted_build_args, monkeypatch, option_n
         assert intercepted_build_selector.build_patterns == build_selector_patterns
     else:
         assert intercepted_build_selector.skip_patterns == build_selector_patterns
-
-

unit_test/main_tests/main_platform_test.py

@@ -1,10 +1,10 @@
-import pytest
-
 import sys
 
+import pytest
+
 from cibuildwheel.__main__ import main
 
-from main_util_fixtures import MOCK_PROJECT_DIR, mock_protection, fake_project_dir, platform, intercepted_build_args
+from conftest import MOCK_PROJECT_DIR  # noqa: I100
 
 
 def test_unknown_platform_non_ci(monkeypatch, capsys):