Makefile: use pytest for code coverage

The command failed with:
coverage run -m unittest discover

----------------------------------------------------------------------
Ran 0 tests in 0.000s

NO TESTS RAN
.../.virtualenvs/pyscard/lib/python3.12/site-packages/coverage/inorout.py:504: CoverageWarning: Module smartcard was never imported. (module-not-imported)
  self.warn(f"Module {pkg} was never imported.", slug="module-not-imported")
.../.virtualenvs/pyscard/lib/python3.12/site-packages/coverage/control.py:894: CoverageWarning: No data was collected. (no-data-collected)
  self._warn("No data was collected.", slug="no-data-collected")

Makefile

@@ -27,7 +27,7 @@ test:
 
 coverage:
 	$(COVERAGE) erase
-	$(COVERAGE) run -m unittest discover
+	$(COVERAGE) run -m pytest
 	$(COVERAGE) combine
 	$(COVERAGE) report
 	$(COVERAGE) html