18799: Mark self parameters as POSITIONAL_OR_KEYWORD

18799: Mark self parameters as POSITIONAL_OR_KEYWORD

On Python 3.7, if you mark a parameter POSITIONAL_ONLY, it will generate
a signature that uses the positional-only `/` syntax, *even though*
Python 3.7 cannot parse that syntax (it was added in Python 3.8, PEP 570).

POSITIONAL_OR_KEYWORD avoids that problem, and is probably more accurate
anyway.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>