Remove the usage of assert in the SPARQLConnector

[white-gecko]

Fixes the usage of assert as introduced in #1175 in the SPARQLConnector.

Proposed Changes

In my eyes it is no good practice to have assert in production code, so it should not be in a library.
The Python reference states:

The current code generator emits no code for an assert statement when optimization is requested at compile time.
https://docs.python.org/3/reference/simple_stmts.html#assert

So I think it should be replaced by if and raise an exception.

There are many more usages of assert which should be replaced at some point as well.

[coveralls]

Coverage decreased (-0.03%) to 75.395% when pulling b42e5a5 on white-gecko:avoid_using_assert_in_sparqlconnector into 7a53c61 on RDFLib:master.

[nicholascar]

Merging since the only test fails are that cannot assign address issue!