The pyodbc team says those two undefined symbols are part of the C++ Standard Library, and would seem to indicate a problem with the build environment. The folks on Ryver helped me narrow it down to my /usr/local/lib/libgcc_s.a. They gave me this work-around "LDFLAGS=-L/QOpenSys/pkgs/lib pip3 install pyodbc", which worked.
It's still unknown what's wrong with my /usr/local/lib/libgcc_s.a and what I can do about it.
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