Use f-strings in the pr_monitor tool (quantumlib#6208)

* pr_monitor - convert to f-strings to satisfy pylint consider-using-f-string

* clean up some redundant fixed strings concatenations

* also pin pytest-cov==3 to unblock CI coverage check which failed
  because pytest-cov==4.1.0 does not see line hits in a subprocess.

Finalizes quantumlib#6171

Author: pavoljuhas

cirq/_compat_test.py

@@ -781,8 +781,7 @@ def test_deprecated_module_deadline_validation():
 
 def _test_broken_module_1_inner():
     with pytest.raises(
-        DeprecatedModuleImportError,
-        match="missing_module cannot be imported. " "The typical reasons",
+        DeprecatedModuleImportError, match="missing_module cannot be imported. The typical reasons"
     ):
         # pylint: disable=unused-import
         import cirq.testing._compat_test_data.broken_ref as br  # type: ignore

cirq/qis/measures.py

@@ -211,7 +211,7 @@ def _numpy_arrays_to_state_vectors_or_density_matrices(
             state2.shape[0] if state2.ndim == 2 else np.prod(state2.shape, dtype=np.int64).item()
         )
         if dim1 != dim2:
-            raise ValueError('Mismatched dimensions in given states: ' f'{dim1} and {dim2}.')
+            raise ValueError(f'Mismatched dimensions in given states: {dim1} and {dim2}.')
         if qid_shape is None:
             qid_shape = (dim1,)
         else:

cirq/qis/states.py

@@ -314,7 +314,7 @@ def density_matrix(
         ValueError: Invalid density matrix.
     """
     if state.ndim != 2 or state.shape[0] != state.shape[1]:
-        raise ValueError('A density matrix must be a square matrix. ' f'Got shape {state.shape}.')
+        raise ValueError(f'A density matrix must be a square matrix. Got shape {state.shape}.')
     dim, _ = state.shape
     if qid_shape is None:
         qid_shape = _infer_qid_shape_from_dimension(dim)