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):