logging: close log_file_handler

While it should be closed in logging's shutdown [1], the following would
still issue a ResourceWarning:

```
import logging

log_file_handler = logging.FileHandler("temp.log", mode="w", encoding="UTF-8")

root_logger = logging.getLogger()
root_logger.addHandler(log_file_handler)
root_logger.removeHandler(log_file_handler)
root_logger.error("error")

del log_file_handler
```

It looks like the weakref might get lost for some reason.

See 92ffe42b45 / #4981
for more information.

1: https://github.com/python/cpython/blob/c1419578a18d787393c7ccee149e7c1fff17a99e/Lib/logging/__init__.py#L2107-L2139

changelog/4988.bugfix.rst

@@ -0,0 +1 @@
+Close logging's file handler explicitly when the session finishes.

src/_pytest/logging.py

@@ -579,6 +579,9 @@ def pytest_sessionfinish(self):
             if self.log_file_handler is not None:
                 with catching_logs(self.log_file_handler, level=self.log_file_level):
                     yield
+                # Close the FileHandler explicitly.
+                # (logging.shutdown might have lost the weakref?!)
+                self.log_file_handler.close()
             else:
                 yield