Bump execnet from 1.5.0 to 1.6.0

Bumps [execnet](https://bitbucket.org/hpk42/execnet) from 1.5.0 to 1.6.0.
- [Changelog](https://bitbucket.org/hpk42/execnet/src/default/CHANGELOG)
- [Commits](https://bitbucket.org/hpk42/execnet/commits)

requirements.txt

@@ -10,7 +10,7 @@ cmarkgfm==0.4.2
 coverage==4.5.2
 cycler==0.10.0
 docutils==0.14
-execnet==1.5.0
+execnet==1.6.0
 future==0.17.1
 idna==2.8
 kiwisolver==1.1.0