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

export CPATH=/opt/local/include

and the python installation succeeded.