MAINT: cleanup unused imports; avoid redefinition of imports

* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

slow calls to np.dtype.name replaced with np.dtype,
mtrand.pyx and _generator.pyx updated,
test test_warns_byteorder updated

before:
%timeit rs.random(): 520 ns ± 33.1 ns per loop
%timeit rg.random(): 6.36 µs ± 222 ns per loop

after:
%timeit rs.random(): 453 ns ± 6.95 ns per loop
%timeit rg.random(): 594 ns ± 9.66 ns per loop

Move flag to parameters

* PEP 8: "Imports should usually be on separate lines"
* Where modified, sort imported modules alphabetically
* Clean-up unused imports from these expanded lines

This implements NEP 34.