setup: bump whoosh minimum version to 2.7.1 to get py3 support

Fix:

- kallithea/tests/functional/test_search_indexing.py:111 TestSearchControllerIndexing.test_repository_tokenization[content-this_should_be_unique_content-1-group/*]

File ".../lib/python3.7/site-packages/whoosh/reading.py", line 241, in expand_prefix
if fn != fieldname or not text.startswith(prefix):
TypeError: startswith first arg must be bytes or a tuple of bytes, not str