Update gmpy2 compatibility to 2.0.0
summations.py: return Piecewise for symbolic geometric sums in finite case (issue 3703)
Merge pull request #1941 from shikil/gmpy2

Update gmpy2 compatibility to 2.0.0
Add Stephen Loo to the AUTHORS/aboutus. Welcome to SymPy!
Merge pull request #1910 from skirpichev/3703-finite-geom-sum

summations.py: return Piecewise for symbolic geometric sums in finite case (issue 3703)
try return a factored expr more often from simplify
denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x

Should this only be done if the base is a Number?
allow factors of coeff to enter powers
keep expr as option before powsimp
mark some solve tests as slow