readthedocs.org: Sync version fails when two version end up with the same slug

When sync_versions is called if there are two verbose_name that endup with the same slug this code breaks:

https://github.com/rtfd/readthedocs.org/blob/ba7623d2ff83b2c88cecbbe55800317f46116d0d/readthedocs/restapi/utils.py#L76-L82

As example,

In [11]: VersionSlugField(populate_from='').slugify('example/')                                                                                                                               
Out[11]: 'example-'

In [12]: VersionSlugField(populate_from='').slugify('/example/')                                                                                                                              
Out[12]: 'example-'

In [13]: VersionSlugField(populate_from='').slugify('example-')                                                                                                                               
Out[13]: 'example-'

Sentry: https://sentry.io/read-the-docs/readthedocscom/issues/746103998/