Avoid a little global state (#1330)

For even clearer interaction with the environment.

beets/library.py

@@ -53,8 +53,6 @@ class PathQuery(dbcore.FieldQuery):
     escape_re = re.compile(r'[\\_%]')
     escape_char = b'\\'
 
-    _is_windows = platform.system() == 'Windows'
-
     def __init__(self, field, pattern, fast=True, case_sensitive=None):
         """Create a path query.
 
@@ -65,7 +63,7 @@ class PathQuery(dbcore.FieldQuery):
 
         # By default, the case sensitivity depends on the platform.
         if case_sensitive is None:
-            case_sensitive = not self._is_windows
+            case_sensitive = platform.system() != 'Windows'
         self.case_sensitive = case_sensitive
 
         # Use a normalized-case pattern for case-insensitive matches.

test/test_query.py

@@ -18,7 +18,6 @@ from __future__ import (division, absolute_import, print_function,
                         unicode_literals)
 
 from functools import partial
-from mock import patch
 
 from test import _common
 from test._common import unittest
@@ -476,11 +475,11 @@ class PathQueryTest(_common.LibTestCase, TestHelper, AssertsMixin):
         self.assert_items_matched(results, ['path item', 'caps path'])
 
         # test platform-aware default sensitivity
-        with patch('beets.library.PathQuery._is_windows', False):
+        with _common.platform_posix():
             q = makeq()
             self.assertEqual(q.case_sensitive, True)
 
-        with patch('beets.library.PathQuery._is_windows', True):
+        with _common.platform_windows():
             q = makeq()
             self.assertEqual(q.case_sensitive, False)