Turn off pypy3compile in postinst script for .DEB

[spapadim]

PyPy3 does not seem to like type annotations in assignments (although it's ok with function/method annotations). As a result, if pypy3 is installed on the system, /usr/bin/pypy3compile is invoked, which spits out the following error during .DEB installation:

Failed to byte-compile /usr/lib/python3/dist-packages/argonone/__init__.py:   
File "/usr/lib/python3/dist-packages/argonone/__init__.py", line 103
    thresholds: List[float] = []
              ^
SyntaxError: invalid syntax

The installed package still works, of course, but the message is rather annoying.