Removal of Babel in 3.15.2 requirements.txt breaks package

https://github.com/openstack/oslo.i18n/commit/0658992b0b9bb8a9ad71ddf962d89d3ede27b95e removed Babel as a dependency of oslo.i18n

However, this package is imported at runtime, see:

https://github.com/openstack/oslo.i18n/blob/master/oslo_i18n/__init__.py#L14

https://github.com/openstack/oslo.i18n/blob/master/oslo_i18n/_gettextutils.py#L24

This is a breaking change; oslo.i18n can no longer be imported without and error, which breaks oslo.concurrency:

  File "./renderer/fonts.py", line 9, in <module>
    from oslo_concurrency import lockutils
  File ".../lib/python2.7/site-packages/oslo_concurrency/lockutils.py", line 33, in <module>
    from oslo_concurrency._i18n import _, _LI
  File ".../lib/python2.7/site-packages/oslo_concurrency/_i18n.py", line 17, in <module>
    import oslo_i18n
  File ".../lib/python2.7/site-packages/oslo_i18n/__init__.py", line 14, in <module>
    from ._gettextutils import *
  File ".../lib/python2.7/site-packages/oslo_i18n/_gettextutils.py", line 24, in <module>
    from babel import localedata
ImportError: No module named babel

I'm not sure what the motivation behind https://github.com/openstack/oslo.i18n/commit/0658992b0b9bb8a9ad71ddf962d89d3ede27b95e was, but it's wrong; babel is a runtime requirement.