Update to latest release

Selected improvements and bug fixes

• The check_units() decorator can now express that some arguments need to have the same units. This mechanism is now used to check the units of the clip() function (#1234). Thanks to Felix Kern for notifying us of this issue.
• Using SpatialNeuron with Cython no longer raises an unnecessary warning when the scipy library is not installed (#1230).
• Raise an error for references to N_incoming or N_outgoing in calls to Synapses.connect. This use is ill-defined and led to compilation errors in previous versions (#1227). Thanks to Denis Alevi for making us aware of this issue.

Infrastructure and documentation improvements

• Brian no longer officially supports installation on 32bit operating systems. Installation via pip will probably still work, but we are no longer testing this configuration (#1232).
• Automatic continuous integration tests for Windows now use the Microsoft Azure Pipeline infrastructure instead of Appveyor. This should speed up tests by running different configurations in parallel (#1233).
• Fix an issue in the test suite that did not handle NotImplementedError correctly anymore after the changes introduced with #1196.