Breaking change in `Cython` - `python-rocksdb` python package dependency causing build issues with pip install of `python-rocksdb`

[WadeBarnes]

This issue was reported in hyperledger/indy-node#1822 however the actual dependency issue exists at the indy-plenum level.

There has been a breaking change to the Cython package between 0.29.36 and the 3.0.x versions. This causes the rocksdb build to fail when it is installed using the python-rocksdb python package. This in turn causes the indy-plenum install to fail when it is installed using the python package. The indy-plenum and rocksdb deb packages built for indy-plenum are not affected, therefore there are no issues when indy-plenum is installed using apt-get.

Fix the Cython - python-rocksdb python package dependency issues. The quick fix for this is to pin the version of Cython to 0.29.36. However, some effort should be made to better understand the issues encountered with the 3.0.x versions of Cython to determine whether something can be done to fix the build when they are used.