remove __pycache__ directories when cleaning python test cases

The python3 generates __pycache__ directories to store *.pyc file within. They should be removed during clean.

##### Before patch:
```
ptomulik@node00$ make -k -j16 PY3=y check-python-examples; (cd Examples/test-suite/python/ && make -s -k -j16 PY3=y check);
[...]
ptomulik@node00$ make clean;
ptomulik@node00$ find . -name '*.pyc'
./Examples/test-suite/python/__pycache__/enum_forward.cpython-32.pyc
./Examples/test-suite/python/__pycache__/array_member.cpython-32.pyc
./Examples/test-suite/python/__pycache__/li_std_pair_using.cpython-32.pyc
./Examples/test-suite/python/__pycache__/nested_workaround.cpython-32.pyc
./Examples/test-suite/python/__pycache__/abstract_typedef2.cpython-32.pyc
./Examples/test-suite/python/__pycache__/using_extend.cpython-32.pyc
./Examples/test-suite/python/__pycache__/li_cwstring.cpython-32.pyc
./Examples/test-suite/python/__pycache__/kwargs_feature.cpython-32.pyc
./Examples/test-suite/python/__pycache__/preproc.cpython-32.pyc
./Examples/test-suite/python/__pycache__/class_scope_weird.cpython-32.pyc
./Examples/test-suite/python/__pycache__/import_nomodule.cpython-32.pyc
[...]
```

##### After patch:
```
ptomulik@node00$ make -k -j16 PY3=y check-python-examples; (cd Examples/test-suite/python/ && make -s -k -j16 PY3=y check);
[...]
ptomulik@node00$ make clean;
ptomulik@node00$ find . -name '*.pyc'
```