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.
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
Drop ancient oseries method
Drop _hashable_content (inherit from Basic)
Add Sushant Hiray to AUTHORS/aboutus/.mailmap. Welcome to SymPy!