Update build_primer_pairs() to accept Iterable[Oligo] or Sequence[Oligo]

[msto]

I have a use case where I have subclassed Oligo in order to store QC attributes (e.g. hairpin delta G and off-target hit counts) and filter individual primers before assembling them into pairs.

Because list is invariant, the current implementation does not support building pairs from subclasses of Oligo.

Note that the implementation prior to #75 supported this use case because the arguments were defined as Iterable[Oligo].

[msto]

https://mypy.readthedocs.io/en/stable/common_issues.html#variance