Concatenation between str and bytes no longer tolerated in Python 3.8.5+

[Abdillah]

This package no longer compatible with Python v3.8.5+ due to the stricter nature of string concatenation.

This PR might be the solution #33.

What happened?

The following error occured using the sample provided in the README.

>>> s.query("SELECT ?q WHERE { ?q ?p ?r } LIMIT 1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/ax9qn8myhf1d2zwa095b3zl2avm1ynwa-python3-3.8.9-env/lib/python3.8/site-packages/sparql.py", line 416, in query
    return q.query(query, timeout, raw=raw)
  File "/nix/store/ax9qn8myhf1d2zwa095b3zl2avm1ynwa-python3-3.8.9-env/lib/python3.8/site-packages/sparql.py", line 571, in query
    response = self._request(statement, timeout)
  File "/nix/store/ax9qn8myhf1d2zwa095b3zl2avm1ynwa-python3-3.8.9-env/lib/python3.8/site-packages/sparql.py", line 558, in _request
    response = self._build_response(query, opener, buf, timeout)
  File "/nix/store/ax9qn8myhf1d2zwa095b3zl2avm1ynwa-python3-3.8.9-env/lib/python3.8/site-packages/sparql.py", line 539, in _build_response
    request = self._build_request(query)
  File "/nix/store/ax9qn8myhf1d2zwa095b3zl2avm1ynwa-python3-3.8.9-env/lib/python3.8/site-packages/sparql.py", line 505, in _build_request
    uri = self.endpoint.strip() + separator + query
TypeError: can only concatenate str (not "bytes") to str

Expected

No error.