revisit linting `noqa`s

[jGaboardi]

revisit linting noqas

• spglm/spglm/glm.py

  Line 343 in 4d435f7

  model = self.model # noqa F841 - `model` never used

• spglm/spglm/base.py

  Line 309 in 4d435f7

  dot_fun = nan_dot # noqa F821 - `nan_dot` not defined - should remove

• spglm/spglm/utils.py

  Line 23 in 4d435f7

  string_types = basestring # noqa F821 - `basestring` not defined - should remove

• spglm/spglm/varfuncs.py

  Line 120 in 4d435f7

  from statsmodels.tools.numdiff import approx_fprime_cs # noqa F401

xref #37