travis: Ensure Python 3.8 is used, disable failing PyPy 2.7 build#2160

EricCousineau-TRI wants to merge 3 commits into

pybind:masterpybind/pybind11:masterfrom

EricCousineau-TRI:issue-2159EricCousineau-TRI/pybind11:issue-2159Copy head branch name to clipboard