SymPyDocTestFinder: don't test functions which are not part of SymPy

these are the modules which are skipped:

array
basic
compiler.pycodegen
compiler.transformer
dense_coding_example
differential_equations
differentiation
expansion
fnmatch
functions
hydrogen
matplotlib.collections
matplotlib.colors
matplotlib.lines
matplotlib.patches
mpl_toolkits.mplot3d.axes3d
partial_differential_eqs
plotting_nice_plot
posixpath
precision
print_gtk
print_pretty
pyglet.clock
pyglet_plotting
re
series
substitution
doctest of memoization functions fix

fix doctest of functions which are decorated with one of the *_memo decorators
SymPyDocTestFinder: add additional check
minor cleanup of SymPyDocTestFinder