Fix N-Wouda#40

Merge pull request N-Wouda#39 from N-Wouda/allow-zero-weights

Allow zero weights

Merge pull request N-Wouda#38 from N-Wouda/py-3.9

Simplify callbacks [+Python 3.9 support]

Unofficial support for np 1.17 rnd.Generator (N-Wouda#26)

* Unofficial support for np 1.17 rnd.Generator

Improve ALNS evaluation order (N-Wouda#25)

* Improve (but change) ALNS evaluation order

* Bump version number

* Fix ordering issue in evaluation

Plot objective legend (N-Wouda#23)

* Objective plot legend

Remove Py3.4 from setup and Travis script

Fix N-Wouda#20 (N-Wouda#21)

Callbacks (N-Wouda#18)

* Title argument for Result.plot_objective

* Call-back mixin for a new global best

* Warn when overwriting callback

* Flexible operator_counts plotting

* Operator decay parameters in [0, 1]

* Tests

v1.1.1

Changelog:
- Test and support for Python 3.8