Don't expose private styles in style.available

They remain in `style.library`, because that's how we look them up, but
this prevents them being exposed as something someone might use.

Also, fix `reload_library`, which was accidentally modifying the
original base library information each time.

Fixes itprojects/MasVisGtk#13

Author: QuLogic

lib/matplotlib/style/__init__.py

@@ -245,8 +245,9 @@ def update_nested_dict(main_dict, new_dict):
 def reload_library():
     """Reload the style library."""
     library.clear()
-    library.update(_update_user_library(_base_library))
-    available[:] = sorted(library.keys())
+    library.update(_update_user_library(_base_library.copy()))
+    available[:] = sorted(name for name in library
+                          if not name.startswith('_') or name not in _base_library)
 
 
 reload_library()

lib/matplotlib/tests/test_style.py

@@ -21,6 +21,7 @@ def temp_style(style_name, settings=None):
     if not settings:
         settings = DUMMY_SETTINGS
     temp_file = f'{style_name}.mplstyle'
+    orig_library_paths = style.USER_LIBRARY_PATHS
     try:
         with TemporaryDirectory() as tmpdir:
             # Write style settings to file in the tmpdir.
@@ -32,6 +33,7 @@ def temp_style(style_name, settings=None):
             style.reload_library()
             yield
     finally:
+        style.USER_LIBRARY_PATHS = orig_library_paths
         style.reload_library()
 
 
@@ -46,8 +48,10 @@ def test_invalid_rc_warning_includes_filename(caplog):
 
 
 def test_available():
+    assert '_classic_test_patch' not in style.available
     with temp_style('_test_', DUMMY_SETTINGS):
         assert '_test_' in style.available
+    assert '_test_' not in style.available
 
 
 def test_use():