Saltclass: broken list expansion

Description of Issue/Question

This used to work in 2018.3.x:

pillars:
  groups:
    - group1
    - group2
  users:
    user1:
      groups: ${groups}

It seems that #47799 broke list expansions. The example above fails on Fluorine with the following exception:

% salt-ssh example.com pillar.items

[ERROR   ] Exception caught loading ext_pillar 'saltclass':
  File "/Users/user/salt/repos/saltstack-repo/salt/pillar/__init__.py", line 951, in ext_pillar
    key)
  File "/Users/user/salt/repos/saltstack-repo/salt/pillar/__init__.py", line 879, in _external_pillar_data
    *val)
  File "/Users/user/salt/repos/saltstack-repo/salt/pillar/saltclass.py", line 64, in ext_pillar
    return sc.get_pillars(minion_id, salt_data)
  File "/Users/user/salt/repos/saltstack-repo/salt/utils/saltclass.py", line 316, in get_pillars
    pillars_dict_expanded = expand_variables(pillars_dict['pillars'], {}, [])
  File "/Users/user/salt/repos/saltstack-repo/salt/utils/saltclass.py", line 176, in expand_variables
    expand_variables(v, b, expanded, path + [six.text_type(k)])
  File "/Users/user/salt/repos/saltstack-repo/salt/utils/saltclass.py", line 176, in expand_variables
    expand_variables(v, b, expanded, path + [six.text_type(k)])
  File "/Users/user/salt/repos/saltstack-repo/salt/utils/saltclass.py", line 186, in expand_variables
    b = find_and_process_re(v, v, k, b, expanded)
  File "/Users/user/salt/repos/saltstack-repo/salt/utils/saltclass.py", line 158, in find_and_process_re
    v_new = v.replace(re_str, v_expanded)

[CRITICAL] Pillar render error: Failed to load ext_pillar saltclass: replace() argument 2 must be str, not list
example.com:
    ----------
    _errors:
        - Failed to load ext_pillar saltclass: replace() argument 2 must be str, not list
    groups:
        - group1
        - group2
    users:
        ----------
        user1:
            ----------
            groups:
                ${groups}

Start in salt/utils/saltclass.py, especially expand_variables and find_and_process_re, using the reported salt-ssh example.com pillar.items command to reproduce the failure. Trace how the groups list is expanded through the users entry. Done means the Saltclass example expands the list successfully without the replace() argument type error.