fix: add _use/_force_indexes into __copy__ with shallow copy

[elisong]

By #334, #470 , pypika got force_index、use_index feature, but forgot to add "_force_indexes", "_use_indexes" into QueryBuilder.__copy__() with shallow copy, thus they both become mutable. For examples

from pypika import Query

query = Query.from_('customers').select('id', 'fname')
q1 = query.use_index('idx1')
q2 = query
q3 = query.use_index('idx2')

if __name__ == "__main__":
    print(q1.get_sql())
    # SELECT "id","fname" FROM "customers" USE INDEX ("idx1","idx2")
    print(q2.get_sql())
    # SELECT "id","fname" FROM "customers" USE INDEX ("idx1","idx2")
    print(q3.get_sql())
    # SELECT "id","fname" FROM "customers" USE INDEX ("idx1","idx2")

[x8lucas8x]

@elisong that looks good, but can you please add tests to ImmutabilityTests, so that this can be tested. So far we only have join tests there (i.e. test_queries_after_join). If you could add test cases for other things that are being copied as well (see copy), such as selects, groupbys, and so on, that would be great (e.g. test_queries_after_groupby, test_queries_after_select).

[coveralls]

Coverage increased (+0.01%) to 98.419% when pulling b9aa50e on elisong:add-use/force-indexes-shallow-copy into 46c90d1 on kayak:master.

[elisong]

@x8lucas8x Fine, I added some tests.

[x8lucas8x]

@elisong thank you. It looks great now.