Raise PlatformMismatchError if using wrong class

Instead of leaving abstract methods without definition when running on
the other platform (Unix vs Windows), define them and raise a
PlatformMismatchError if called on the wrong platform.

This fixes pylint warnings such as
   Abstract class 'WindowsFileLock' with abstract methods instantiated
   Abstract class 'UnixFileLock' with abstract methods instantiated

Fixes tox-dev#102

src/filelock/_api.py

@@ -14,6 +14,10 @@
 _LOGGER = logging.getLogger("filelock")
 
 
+class PlatformMismatchError(Exception):
+    """Exception raised when attempting Unix locks on Windows, or vice versa."""
+
+
 # This is a helper class which is returned by :meth:`BaseFileLock.acquire` and wraps the lock to make sure __enter__
 # is not called twice when entering the with statement. If we would simply return *self*, the lock would be acquired
 # again in the *__enter__* method of the BaseFileLock, but not released again automatically. issue #37 (memory leak)
@@ -236,4 +240,5 @@ def __del__(self) -> None:
 __all__ = [
     "BaseFileLock",
     "AcquireReturnProxy",
+    "PlatformMismatchError",
 ]

src/filelock/_unix.py

@@ -5,15 +5,21 @@
 from abc import ABC
 from typing import cast
 
-from ._api import BaseFileLock
+from ._api import BaseFileLock, PlatformMismatchError
 
 #: a flag to indicate if the fcntl API is available
 has_fcntl = False
 if sys.platform == "win32":  # pragma: win32 cover
 
-    class UnixFileLock(BaseFileLock, ABC):
+    class UnixFileLock(BaseFileLock):
         """Uses the :func:`fcntl.flock` to hard lock the lock file on unix systems."""
 
+        def _acquire(self) -> None:
+            raise PlatformMismatchError()
+
+        def _release(self) -> None:
+            raise PlatformMismatchError()
+
 else:  # pragma: win32 no cover
     try:
         import fcntl

src/filelock/_windows.py

@@ -6,7 +6,7 @@
 from errno import ENOENT
 from typing import cast
 
-from ._api import BaseFileLock
+from ._api import BaseFileLock, PlatformMismatchError
 from ._util import raise_on_exist_ro_file
 
 if sys.platform == "win32":  # pragma: win32 cover
@@ -49,9 +49,15 @@ def _release(self) -> None:
 
 else:  # pragma: win32 no cover
 
-    class WindowsFileLock(BaseFileLock, ABC):
+    class WindowsFileLock(BaseFileLock):
         """Uses the :func:`msvcrt.locking` function to hard lock the lock file on windows systems."""
 
+        def _acquire(self) -> None:
+            raise PlatformMismatchError()
+
+        def _release(self) -> None:
+            raise PlatformMismatchError()
+
 
 __all__ = [
     "WindowsFileLock",