Add sympy to requirements

SymPy is a Python library for symbolic mathematics.
It is used by the Vitrage project for template parsing.
Sympy is depended upon mpmath add it only to upper-constraints

- Is the library actively maintained?
  Yes.
  here: https://github.com/sympy/sympy

- Is the library good code?
  Yes.
  It is stable and runs tests on the different actions it performs.
  We use it in Vitrage project.
  Vitrage: https://wiki.openstack.org/wiki/Vitrage

- Is the library python 3 compatible?
  Yes.

- Is the library license compatible?
  Yes, BSD.

- Is the library already packaged in the distros we target (Ubuntu
  latest / Fedora latest)?
  Yes.

- Is the function of this library already covered by other libraries
  in ``global-requirements.txt``?
  No.

Change-Id: Ic2558b3e13810e9787f3dd21f323f20a8adec798

global-requirements.txt

@@ -233,6 +233,7 @@ sqlalchemy-utils  # BSD License
 stevedore>=1.10.0  # Apache-2.0
 sysv_ipc>=0.6.8  # BSD License
 suds-jurko>=0.6  # LGPL
+sympy>=0.7.6  # BSD
 taskflow>=1.26.0  # Apache-2.0
 tempest>=11.0.0  # Apache-2.0
 tempest-lib>=0.14.0  # Apache-2.0

upper-constraints.txt

@@ -160,6 +160,7 @@ mock===2.0.0
 monotonic===1.1
 mox===0.5.3
 mox3===0.16.0
+mpmath===0.19
 msgpack-python===0.4.7
 netaddr===0.7.18
 netifaces===0.10.4
@@ -331,6 +332,7 @@ sqlalchemy-migrate===0.10.0
 sqlparse===0.1.19
 stevedore===1.15.0
 suds-jurko===0.6
+sympy===0.7.6
 sysv-ipc===0.7.0
 tablib===0.11.2
 taskflow===2.1.0