make the exec plugin fully py3 compatible

resolves #99
v0.10.28
adding jedi_linter plugin

Resolves #100
Merge pull request #101 from pybuilder/feature/jedi_linter

adding jedi_linter plugin
v0.10.29
resolving problems with the jedi plugin

* improving the message when backwards compatibility was broken
* undoing jedi's chdir() by moving back into the project root after
  analysis
v0.10.30
add jedi traceback to debug output
fix misleading error message

Failing to install a dependency which is a simple string (and not a
Dependency object) yields the strange error "str object has no attribute
name". This has now been resolved.
v0.10.31
fixing error message when jedi is missing
implement using requirements files as dependencies

This resolves #96.
A `Project` exposes two new methods, `depends_on_requirements` and
`build_depends_on_requirements` which can be used to include a
file of type `requirements.txt` in the corresponding dependencies.

Note that runtime requirements will be added to the setup.py script
so that runtime dependencies are pulled in.
fix python3 issue
WTF flake8, this is for py3 compat