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

setup.py

@@ -53,7 +53,7 b' requirements = ['
     "SQLAlchemy >= 1.1, < 1.4",
     "Mako >= 0.9.1, < 1.1",
     "Pygments >= 2.2.0, < 2.5",
-    "Whoosh >= 2.5.0, < 2.8",
+    "Whoosh >= 2.7.1, < 2.8",
     "celery >= 3.1, < 4.0", # TODO: celery 4 doesn't work
     "Babel >= 1.3, < 2.8",
     "python-dateutil >= 2.1.0, < 2.9",