• Change: When an env parameter is specified to the spawn() or
  run() family of calls containing a value for PATH, its value is
  used to discover the target executable from a relative path, rather than the
  current process's environment PATH. This mirrors the behavior of
  subprocess.Popen in the standard library (PR #348).
• Regression: Re-introduce capability for method read_nonblocking() in class
  fdspawn as previously supported in version 3.3 (PR #359).