Merge pull request #18 from graph-algorithms/Issue15-GuardUnistdInclude

Closes #15

A separate issue is needed to remove the failing github action, because Python 3.7 past end-of-life (so ignoring the failure of that github action in this PR merge).

Issue #15 - Guard `#include <unistd.h>` in `planarityCommandLine.c` based on compiler

Author: john-boyer-phd