Fix pycontracts incompatibility with pyparsing v3

[adatzer]

As discussed in this Discourse post, pycontracts v1.8, that the tracker depends on, is incompatible with the recently released pyparsing, because of the deprecated operatorPrecedence that was removed in pyparsing v3.
This means that in Python 3 environments, pyparsing v3 will be installed as an indirect dependency of the tracker, resulting in error.

[DigUpTheHatchet]

Ada Tzereme (@adatzer) Do you have an idea of what would be the best solution to resolve this issue?

[adatzer]

Hi Dylan Kelly (@DigUpTheHatchet) , we've just opened #257 with a possible solution. Until then, another quick fix to consider is also described in the respective pycontracts' issue, that involves explicitly pinning pyparsing to an earlier version.