add pypy3 to build matrix

excluding on linux until the ubuntu ppa has it
also exclude 0.9.8 pypy3 on linux for now
add a comment explaining why we're excluding pypy3 on linux
don't do coverage under pypy3
Combined duplicated serialization tests
Merge pull request #1571 from reaperhulk/add-pypy3

add pypy3 to build matrix
Merge pull request #1572 from alex/delete-duplicate

Combined duplicated serialization tests
final cleanup for PEM serialization