test_type_G.py uses utf-8 characters and should therefore specify the
proper encoding
added various from __future__ statements to placate pytest
Antidivisors

Implemented functions to compute antidivisors and the number of
antidivisors.  Antidivisors are the numbers that do not divide a number
by the largest possible margin [1]_.

References
==========

.. [1] http://oeis.org/A066272/a066272a.html
Implement divisor_sigma function

Divisor function [1]_ sigma_k(n) for positive integer n:

divisor_sigma(n,0) is equal to divisor_count(n)
divisor_sigma(n,1) is equal to sum(divisors(n))
divisor_sigma(n,k) is equal to sum([x**k for x in divisors(n)])

Default for k is 1.

References
==========

.. [1] http://en.wikipedia.org/wiki/Divisor_function
Merge pull request #8174 from pbrady/fix_test_type_G_encoding

fix encoding error in test_type_G.py
Merge pull request #8180 from pbrady/fix_test_type_F

Added various from __future__ statements to placate pytest
integration inifite limits fix
Merge pull request #8185 from skirpichev/sigma-rebase

Implement divisor_sigma function
Merge pull request #8184 from skirpichev/antidivisors-rebase

Antidivisors (rebase of #7863)
Merge pull request #8182 from leonidbl91/integrate_infinite_fix

integration inifite limits fix
Move limit() body to Limit.doit() method

Now limit(*args) is a synonim for
Limit(*args).doit(deep=False), like summation() vs Sum.
For limits at oo/-oo the dir argument doesn't matter:

in this case it's determined from the direction of the infinity.
pretty/latex printing support for directional limits

fixes #8175
Add support for bidirectional limits (dir="real")