After upgrading to macOS BigSur, I was unable to compile python packages which required header files from MacPorts:
In file included from src/blob.c:30: src/blob.h:33:10: fatal error: 'git2.h' file not found #include <git2.h> ^~~~~~~~ 1 error generated. error: command '/usr/bin/clang' failed with exit status 1
The solution was to modify the compiler's include path to include the MacPorts header directory
and the python installation succeeded.