Changed lambdastr to dummy symbols if dummify=True

Previously, if the user declared alpha = symbols('\alpha'),
lambdastr would try to use \alpha as a variable. Now all
all symbols are replaced with dummies if dummify is true,
so any string is a valid variable name.

If the user wants to not use Dummy symbols, they can still
set dummify=False, and get direct substitution. This allows
the output to be examined, but will result in a syntax error
in lambdify if a non-valid symbol name is used (such as \alpha)
fix Dummy.sort_key and use it in expr.sort_key

Also, catch any new Numbers that are generated by the rebuilding
of terms in Mul.flatten. Rather than check them at each step of
the way, just handle them once when done traversing the args.
Add early exit in the limit heuristics() for "bad" results with oo

    This does limit calculations in this method more
    fragle, which is good.  An example:
    >>> O(sin(1/x**2).contains(O(cos(1/x**2)))
    True
    was wrong, just because of this limit:
    >>> limit(sin(1/x**2)/cos(1/x**2), x, 0)
    sin(oo)/cos(oo)
Fix #7282 by replacing the shutdown_hook with the atexit module.
Add an extra config to sphinx to fix #6618.
DocFix: Replaced import mpmath with import sympy.mpmath
Merge pull request #7239 from smichr/dsort

fix Dummy.sort_key and use it in expr.sort_key
Merge pull request #7295 from nebffa/issue_7282

Fix #7282
Merge pull request #7298 from nebffa/issue_6618

Fix #6618.
Merge pull request #7312 from debugger22/master

DocFix: Replaced import mpmath with import sympy.mpmath
Merge pull request #2928 from skirpichev/contains-use-gruntz

Add early exit in the limit heuristics() for "bad" results with oo
Fix .mailmap for Jim Zhang
Add Patrick Poitras to .mailmap
Fix .mailmap for Thomas Hisch
Add Sushant Hiray to AUTHORS/aboutus/.mailmap. Welcome to SymPy!
Fix .mailmap for Anurag Sharma
Add Abhinav Chanda to the AUTHORS/aboutus. Welcome to SymPy!
Add Jim Crist to AUTHORS/aboutus/.mailmap. Welcome to SymPy!
Add Sudhanshu Mishra to the AUTHORS/aboutus. Welcome to SymPy!
Add Soumya Dipta Biswas to the AUTHORS/aboutus. Welcome to SymPy!