We are using shlex.split to parse aliases, which defaults to a POSIX mode,

which strips quotes and evaluates escape sequences. This lead to issues

where an alias of the form !find . -iname '*.pyc' -delete would be

evaluated into find . -iname *.pyc -delete run by the shell, which

would result in the *.pyc being evaluated by the shell instead of by

find.
Now we operate in POSIX mode only when we are not passing the command

to the shell so shell-specific sequences will be untouched. This results

in aliases not needing double escaping when being passed to the shell.
When not passing to the shell, we still parse in POSIX mode to remove

quotes, etc. because otherwise subprocess.call() will pass the quoted

strings as arguments (e.g., ['cmd', '"arg with spaces"'] instead of

['cmd', 'arg with spaces']).
While I was here, I also modernized the docstrings of all the alias

methods to our doc standards. replace_arguments also now returns a

list instead of a generator because we were always immediately wrapping

it in a call to list anyway.

Ran unit tests.
Ran rbt clear-pycs with .pyc files in the current directory. All

.pyc files were recursively removed.