Use context manager instead of plain open (#1211)

Context manager with open closes the files after usage.

When the object is already a pathlib.Path i used read_text
write_text functions

In some cases pathlib.Path.open were used in context manager,
it is basically the same as builtin open.

Thanks, Konstantin Baikov!

update-docs-help.py

@@ -18,7 +18,7 @@ def update_help_includes():
         result = runner.invoke(cli, name.split() + ["--help"], terminal_width=88)
         actual = f"$ datasette {name} --help\n\n{result.output}"
         actual = actual.replace("Usage: cli ", "Usage: datasette ")
-        open(docs_path / filename, "w").write(actual)
+        (docs_path / filename).write_text(actual)
 
 
 if __name__ == "__main__":