Avoid subprocess window creation with agraph.AGraph._run_prog

[chrizzFTD]

Hi pygraphviz team,

Thanks for this great project!

Context

I'm using pygraphviz through networkx for some python widgets that are added to a host application in the form of a plugin.

In Windows OS, running networkx.drawing.pygraphviz_layout leads to a terminal window appearing and disappearing very quickly. This seems to happen exactly during execution of pygraphviz.agraph.AGraph._run_prog on the subprocess call:

pygraphviz/pygraphviz/agraph.py

Lines 1381 to 1388 in fce4667

	p = subprocess.Popen(
	dotargs,
	shell=False,
	stdin=subprocess.PIPE,
	stdout=subprocess.PIPE,
	stderr=subprocess.PIPE,
	close_fds=False,
	)

Changes in this PR

This PR proposes that if subprocess.CREATE_NO_WINDOW is available (Python-3.7+ & Windows OS), then it will be passed as a creation flag to the subprocess. The python docs state this flag is for:

A Popen creationflags parameter to specify that a new process will not create a window.

An example with this change with the python widgets I'm using (the top left selection triggers the networkx call, and on the bottom section we see the graph GUI that has been created through pygraphviz):

Before this PR	With this PR

---

I aimed at making the changes not invasive, though I'd be happy to make adjustments that you see fit; please let me know your thoughts,

Thanks!

-Chris