Fix dev server crash caused by Tailwind exiting immediately

Python's multiprocessing with spawn mode (default on macOS) sets child
stdin to /dev/null. Tailwind's --watch mode monitors stdin and exits on
EOF, so it would exit immediately after compiling, bringing down all
other dev server processes.

Pipe stdin instead so it stays open for the lifetime of the parent.

Author: davegaeddert

plain-dev/plain/dev/poncho/process.py

@@ -97,6 +97,7 @@ class Popen(subprocess.Popen):
     def __init__(self, cmd: str, **kwargs: Any) -> None:
         start_new_session = kwargs.pop("start_new_session", True)
         options = {
+            "stdin": subprocess.PIPE,
             "stdout": subprocess.PIPE,
             "stderr": subprocess.PIPE,
             "shell": True,