use string.ascii_letters for Python 3
Merge pull request #1802 from jrioux/matrices

solve: handle Matrix equations.
Merge pull request #1790 from jrioux/solvers

solve: return all independent solutions.
Merge branch 'master' into remove-old-wiki-refs
Drop references to
https://github.com/sympy/sympy/wiki/old-wiki-Sympy-Development
Drop obsoleted refs to
https://github.com/sympy/sympy/wiki/old-wiki-Discussion
Try to fix Sphinx error
Merge pull request #1812 from smichr/poly-coeffs

Poly coeffs
Merge pull request #1808 from smichr/symbols

3576: symbols handles multiple ranges
skip SinglePSpace test_args
dict comprehension to dict(tuple generator)
use subprocess module in preview.py instead of os.system
make preview function threadsafe again
added support for a few new kwargs to the preview function + cleanup
use more kwargs for SymPyDeprecationWarning
updated docstring
reworded docstring
fix test failures
removed call to sleep()
fix doctest error

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>