nmrpack install fails with unknown variable capture_output

[varioustoxins]

this is a bug in environment.py  get_environment_change which uses sub_process_run capture_output which isn't available in python3.6 which is the install python3 on centos8

required change is below

#pre = subprocess.run(['/usr/bin/env', '-i', '/bin/csh', '-c', f"{pre_command}"], capture_output=True)
#post = subprocess.run(['/usr/bin/env', '-i', '/bin/csh','-c', f"{post_command}"], capture_output=True)

# required for python 3.6 
pre = subprocess.run(['/usr/bin/env', '-i', '/bin/csh', '-c', f"{pre_command}"], stdout=PIPE, stderr=PIPE)
post = subprocess.run(['/usr/bin/env', '-i', '/bin/csh','-c', f"{post_command}"], stdout=PIPE, stderr=PIPE)