added condition in strong pseudoprime
added ``sqrt_mod, quadratic_residue, primitive_root, nthroot_mod``
added `is_nthpow_residue``; faster ``n_order``
added case in ``nthroot_mod``
fixed failures in Python3
fixed bug in  ``_sqrt_mod1``
added new functions in ``ntheory.rst``
fixed bug in  ``_sqrt_mod1``
fixed misprint
fixed bug in ``primitive_root``; increased coverage
fixed bugs in ``primitive_root``
fixed bug in primitive_root
eliminated default argument ``all_roots`` in ``primitive_root``;
added ``_primitive_root_prime_iter``
improved ``n_order`` and ``primitive_root`` in the case of large modulus.
added default argument ``limit``  to ``sqrt_mod``
some documentation change
small changes in ``sqrt_mod`` and ``_sqrt_mod_prime_power``
fixed bug in ``quadratic_residues``; small change in ``_sqrt_mod_prime_power``;
added comments and a test.
faster ``is_quad_residue``;
in ``sqrt_mod`` changed default value of ``limit`` to 1;
changed a test and added another one
added ``sqrt_mod_iter``
``sqrt_mod(a, p)`` returns a root ``x <= p // 2``