Describe the bug
Calling setup.py with clean as argument should clean up the build directories. However, it currently causes the cythonization of ot/lp/emd_wrap.pyx, leaving the build directory unclean. This is a problem for downstream distributions.

To Reproduce
Steps to reproduce the behavior:

1. Check out a clean copy of the code.
2. Observe there is no ot/lp/emd_wrap.cpp.
3. Run python3 setup.py clean.

Expected behavior
A clean build directory.

Actual outcome

$ python3 setup.py clean
Compiling ot/lp/emd_wrap.pyx because it changed.
[1/1] Cythonizing ot/lp/emd_wrap.pyx
/usr/lib/python3/dist-packages/Cython/Compiler/Main.py:369: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: /home/gspr/tmp/POT/ot/lp/emd_wrap.pyx
  tree = Parsing.p_module(s, pxd, full_module_name)
running clean

$ head -n 1 ot/lp/emd_wrap.cpp
/* Generated by Cython 0.29.14 */

• OS: Debian 11 (Testing)
• Python version: 3.7.6
• How was POT installed: source, both 1b58440 and 0.6.0 were tested