'PURE_PYTHON=1' ignored during build

[tseaver]

The C extension gets built willy-nilly. This is after removing usedevelop = true from tox.ini (see zopefoundation/meta#257):

$ ll .tox/py38-pure/lib/python3.8/site-packages/zodbpickle/
total 964
drwxrwxr-x  4 tseaver tseaver   4096 Jun 27 17:21 ./
drwxrwxr-x 17 tseaver tseaver   4096 Jun 27 17:21 ../
-rw-rw-r--  1 tseaver tseaver    891 Jun 27 17:21 fastpickle.py
-rw-rw-r--  1 tseaver tseaver     44 Jun 27 17:21 __init__.py
-rw-rw-r--  1 tseaver tseaver 205193 Jun 27 17:21 _pickle_33.c
-rw-rw-r--  1 tseaver tseaver  52163 Jun 27 17:21 pickle_3.py
-rwxrwxr-x  1 tseaver tseaver 598232 Jun 27 17:21 _pickle.cpython-38-x86_64-linux-gnu.so*
-rw-rw-r--  1 tseaver tseaver     24 Jun 27 17:21 pickle.py
-rw-rw-r--  1 tseaver tseaver  87531 Jun 27 17:21 pickletools_3.py
drwxrwxr-x  2 tseaver tseaver   4096 Jun 27 17:21 __pycache__/
-rw-rw-r--  1 tseaver tseaver   1061 Jun 27 17:21 slowpickle.py
drwxrwxr-x  3 tseaver tseaver   4096 Jun 27 17:21 tests/

Note that we have tests which rely on this (undesirable) behavior.