tests: Ignore unused Textual snapshots on 3.7

The last version of `pytest-textual-snapshot` to support Python 3.7 uses
a different naming convention for its snapshot results than the current
version. Because of this, once our test suite is updated to use the
naming convention required by the latest version, the old version sees
all of these new snapshot result files as extra junk not being used by
the test suite, and believes they should be removed. Hack around this by
programmatically enabling Syrupy's `--snapshot-warn-unused` option for
Python 3.7.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

tests/conftest.py

@@ -1,4 +1,5 @@
 import socket
+import sys
 
 import pytest
 
@@ -10,3 +11,9 @@ def free_port():
     port_number = s.getsockname()[1]
     s.close()
     return port_number
+
+
+if sys.version_info < (3, 8):
+    # Ignore unused Textual snapshots on Python 3.7
+    def pytest_configure(config):
+        config.option.warn_unused_snapshots = True