Bug#937792: Blocked by reverse dependencies

[Martin Kelly]

We are blocked by the following reverse dependencies:
$ apt-cache rdepends python-gmpy2
python-gmpy2
Reverse Depends:
  python-mpmath
 |python-gmpy2-doc
 |python-gmpy2-common
  pyecm

pyecm is already fixed to use Python 3 and the other two are built by us 
and just need renaming.

Added affects/blocks to python-mpmath.