Fix flake8 complaints

What:
1. Add exception for E203 whitespace before ':'
2. Ignore use of bad variable name "l"
3. Remove unused import

Why:
1. There seems to be a discrepancy between the use of ':' within a slice:
PyCQA/pycodestyle#373. Since black can handle the
whitespace regarding ':' for both in and out of slices, we ignore this error and
trust black to format it.
2. Because l is comonly used in equations for density matrices

tests/test_ham_unrestricted_base.py

@@ -114,7 +114,7 @@ def test_orb_rotate_jacobi():
 
     theta = 2 * np.pi * (np.random.random() - 0.5)
 
-    Test = disable_abstract(BaseUnrestrictedHamiltonian)
+    Test = disable_abstract(BaseUnrestrictedHamiltonian)  # noqa: N806
     ham = Test([one_int_alpha, one_int_beta], [two_int_aaaa, two_int_abab, two_int_bbbb])
 
     with pytest.raises(TypeError):

tests/test_solver_equation.py

@@ -51,7 +51,7 @@ def template_params(self):
 def check_cma():
     """Check if cma module is available."""
     try:
-        import cma
+        import cma  # noqa: F401
     except ModuleNotFoundError:
         return False
     else:

tox.ini

@@ -123,6 +123,8 @@ ignore =
     W503,
     # W504 : Line break occurred after a binary operator
     W504,
+    # E203 : Whitespace before ':'
+    E203,
 
 # pylintrc
 [FORMAT]

wfns/ham/density.py

@@ -335,7 +335,7 @@ def density_matrix(
             # if double excitation
             elif len(left_diff) == 2:
                 i, j = left_diff
-                k, l = right_diff
+                k, l = right_diff  # noqa: E741
                 add_two_density(two_densities, i, j, k, l, val, orbtype=orbtype)
                 add_two_density(two_densities, j, i, l, k, val, orbtype=orbtype)
                 add_two_density(two_densities, k, l, i, j, val, orbtype=orbtype)

wfns/ham/unrestricted_chemical.py

@@ -2,7 +2,6 @@
 import numpy as np
 from wfns.backend import slater, math_tools
 from wfns.ham.unrestricted_base import BaseUnrestrictedHamiltonian
-from wfns.ham.generalized_chemical import GeneralizedChemicalHamiltonian
 
 
 class UnrestrictedChemicalHamiltonian(BaseUnrestrictedHamiltonian):