LSPFileBuffer: Ensure that file contents are never stale by monitoring file modification date

For every read access on .contents, check for external file modification
and re-read if necessary, invalidating the content hash as well.

Fixes #1013

test/solidlsp/python/test_python_basic.py

@@ -16,7 +16,7 @@ from solidlsp.ls_config import Language
 
 
 @pytest.mark.python
-class TestLanguageServerBasics:
+class TestPythonLanguageServerBasics:
     """Test basic functionality of the language server."""
 
     @pytest.mark.parametrize("language_server", [Language.PYTHON], indirect=True)