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!

tests/test_cli_serve_get.py

@@ -14,7 +14,8 @@ def test_serve_with_get(tmp_path_factory):
 
         @hookimpl
         def startup(datasette):
-            open("{}", "w").write("hello")
+            with open("{}", "w") as fp:
+                fp.write("hello")
     """.format(
                 str(plugins_dir / "hello.txt")
             ),