clean all .py files generated by python-test-suite

After ``make && make check-python-test-suite && make clean`` some generated files are left in ``Examples/test-suite/python``:

```
ptomulik@barakus:$ git status
# On branch tests_clean
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       Examples/test-suite/python/clientdata_prop_a.py
#       Examples/test-suite/python/clientdata_prop_b.py
#       Examples/test-suite/python/import_stl_a.py
#       Examples/test-suite/python/import_stl_b.py
#       Examples/test-suite/python/imports_a.py
#       Examples/test-suite/python/imports_b.py
#       Examples/test-suite/python/mod_a.py
#       Examples/test-suite/python/mod_b.py
#       Examples/test-suite/python/multi_import_a.py
#       Examples/test-suite/python/multi_import_b.py
#       Examples/test-suite/python/packageoption_a.py
#       Examples/test-suite/python/packageoption_b.py
#       Examples/test-suite/python/packageoption_c.py
nothing added to commit but untracked files present (use "git add" to track)
```

This patch adds these files to clean target.