Remove Python 2 shims (#1260)

Remove legacy Python 2 compatibility code. Since Python 3 is the only
supported version now, these shims are no longer necessary.

Author: mhucka

src/openfermion/chem/molecular_data.py

@@ -31,12 +31,6 @@
     + 0.5 * \sum_{p,q,r,s} h[p,q,r,s] a_p^\dagger a_q^\dagger a_r a_s
 """
 
-# Define a compatible basestring for checking between Python 2 and 3
-try:
-    basestring
-except NameError:  # pragma: no cover
-    basestring = str
-
 
 # Define error objects which inherit from Exception.
 class MoleculeNameError(Exception):
@@ -248,7 +242,7 @@ def name_molecule(geometry, basis, multiplicity, charge, description):
     Raises:
         MoleculeNameError: If spin multiplicity is not valid.
     """
-    if not isinstance(geometry, basestring):
+    if not isinstance(geometry, str):
         # Get sorted atom vector.
         atoms = [item[0] for item in geometry]
         atom_charge_info = [(atom, atoms.count(atom)) for atom in set(atoms)]
@@ -501,7 +495,7 @@ def __init__(
 
         # Metadata fields with default values.
         self.charge = charge
-        if not isinstance(description, basestring):
+        if not isinstance(description, str):
             raise TypeError("description must be a string.")
         self.description = description
 
@@ -518,7 +512,7 @@ def __init__(
                 self.filename = data_directory + '/' + self.name
 
         # Attributes generated automatically by class.
-        if not isinstance(geometry, basestring):
+        if not isinstance(geometry, str):
             self.n_atoms = len(geometry)
             self.atoms = sorted(
                 [row[0] for row in geometry], key=lambda atom: periodic_hash_table[atom]
@@ -707,7 +701,7 @@ def save(self):
         with h5py.File("{}.hdf5".format(tmp_name), "w") as f:
             # Save geometry (atoms and positions need to be separate):
             d_geom = f.create_group("geometry")
-            if not isinstance(self.geometry, basestring):
+            if not isinstance(self.geometry, str):
                 atoms = [numpy.bytes_(item[0]) for item in self.geometry]
                 positions = numpy.array([list(item[1]) for item in self.geometry])
             else:

src/openfermion/testing/testing_utils.py

@@ -338,17 +338,9 @@ def module_importable(module):
         bool
 
     """
-    import sys
+    from importlib import util
 
-    if sys.version_info >= (3, 4):
-        from importlib import util
-
-        plug_spec = util.find_spec(module)
-    else:
-        # Won't enter unless Python<3.4, so ignore for testing
-        import pkgutil  # pragma: ignore
-
-        plug_spec = pkgutil.find_loader(module)  # pragma: no cover
+    plug_spec = util.find_spec(module)
     if plug_spec is None:
         return False
     else: