remove unused import and fix error test

tests/test_basic_regression.py

@@ -3,7 +3,6 @@ import logging
 import os
 import re
 from pathlib import Path
-from io import StringIO
 import pytest
 from numpy.testing import assert_almost_equal
 from propka.parameters import Parameters

tests/test_streamio.py

@@ -113,8 +113,7 @@ def test_valuerror_notpdb(tmpdir):
 
     ref_path, pdb_path = get_paths(pdb)
 
-    with open(pdb_path, 'r') as writer:
-        filestream = StringIO()
+    filestream = StringIO()
 
     errmsg = "The pdb file does not seem to contain any "
     with pytest.raises(ValueError, match=errmsg):