Read from data socket to avoid "[SSL] shutdown while in init" exception
during shutdown of the dummy server.
Signed-off-by: Christian Heimes <christian@python.org>

<!-- issue-number: [bpo-34391](https://www.bugs.python.org/issue34391) -->
https://bugs.python.org/issue34391
<!-- /issue-number -->
(cherry picked from commit 1590c393)
Co-authored-by: Christian Heimes <christian@python.org>