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 https://github.com/pytest-dev/pytest/pull/4981/commits/92ffe42b45 / #4981
for more information.

1: c1419578a1/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

@@ -577,8 +577,15 @@ class LoggingPlugin(object):
             if self.log_cli_handler:
                 self.log_cli_handler.set_when("sessionfinish")
             if self.log_file_handler is not None:
-                with catching_logs(self.log_file_handler, level=self.log_file_level):
-                    yield
+                try:
+                    with catching_logs(
+                        self.log_file_handler, level=self.log_file_level
+                    ):
+                        yield
+                finally:
+                    # Close the FileHandler explicitly.
+                    # (logging.shutdown might have lost the weakref?!)
+                    self.log_file_handler.close()
             else:
                 yield