_env param does not work with dict-like objects

[saytosid]

sh/sh.py

Lines 1149 to 1151 in 114e6ee

	if not isinstance(env, dict):
	invalid.append(("env", "env must be a dict. Got {!r}".format(env)))
	return invalid

wrt the above snippet, is it necessary to enforce dict? This breaks compatibility with other dict-like objects. eg: os.environ returns a dict-like object os._Environ.

Perhaps a check for isinstance(env, collections.Mapping) would allow for more general interface?

[ecederstrand]

That sounds very reasonable.