IPython.utils._process_posix: import pexpect only when needed

[mkoeppe]

Delaying the import of pexpect to the first use.

This is needed to make import IPython work on Pyodide; see pyodide/pyodide#4452

[hoodmane]

I'd suggest adding a new file making a _process_emscripten:

def system(cmd):
    raise OSError("Not available")

def getoutput(cmd):
    raise OSError("Not available")

def check_pid(cmd):
    raise OSError("Not available")

def arg_split(s, posix=False, strict=True):
    """This one could be made to work but it's not clear if it would be useful..."""
    raise OSError("Not available")

and then apply this patch:

--- a/IPython/utils/process.py
+++ b/IPython/utils/process.py
@@ -15,6 +15,8 @@
     from ._process_win32 import system, getoutput, arg_split, check_pid
 elif sys.platform == 'cli':
     from ._process_cli import system, getoutput, arg_split, check_pid
+elif sys.platform == "emscripten":
+    from ._process_emscripten import system, getoutput, arg_split, check_pid
 else:
     from ._process_posix import system, getoutput, arg_split, check_pid

[Carreau]

I'm going to make a release soon, so I'm going to merge this, but +1 to extend with adding a _process_emscripten.py

[hoodmane]

Thanks @Carreau and @mkoeppe!