Bump cvxopt from 1.2.1 to 1.2.7

Bumps [cvxopt](http://cvxopt.org) from 1.2.1 to 1.2.7.

---
updated-dependencies:
- dependency-name: cvxopt
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -5,7 +5,7 @@ matplotlib
 # Optionals
 # =========
 # for the zonotope MCMC sampler
-cvxopt==1.2.1
+cvxopt==1.2.7
 # for notebook on sampling spanning trees
 networkx 
 # for the documentation

setup.py

@@ -102,7 +102,7 @@
     # trees: To play with uniform spanning trees
     # docs: Documentation bibligraphy and theme theme
     extras_require={
-        'zonotope': ['cvxopt==1.2.1'],
+        'zonotope': ['cvxopt==1.2.7'],
         'trees': ['networkx'],
         'docs': ['sphinxcontrib-bibtex', 'sphinx_rtd_theme']
         }