ci: update pytest

Some jobs were failing with an error that seems to be raised by an
outdated pytest with a newer pytest-rerunfailures. We let pip choose...

```
2022-12-28T16:41:10.1173259Z   File "/tmp/tmp_82tmdi_/lib/python3.7/site-packages/pytest_rerunfailures.py", line 7, in <module>
2022-12-28T16:41:10.1173840Z     from _pytest.resultlog import ResultLog
2022-12-28T16:41:10.1174387Z ModuleNotFoundError: No module named '_pytest.resultlog'
```

.ci/test.py

@@ -22,7 +22,7 @@ exclude = [
     'feeadjuster'
 ]
 global_dependencies = [
-    'pytest==5.*',
+    'pytest',
     'pytest-xdist',
     'pytest-timeout',
     'pytest-rerunfailures',