get_success_url raises an exception when success_url is lazy object

[Tomáš Ehrlich]

In #24133 was added deprecation warning when self.success_url contains old %-style Python formatting strings.

However, success_url could be a lazy object like reverse_lazy and trying to call re.search on lazy object results in TypeError:

../../../../.envs/lingui-next/lib/python3.4/site-packages/django/views/generic/edit.py:166: in get_success_url
    if re.search(r'%\([^\)]+\)', self.success_url):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

pattern = '%\\([^\\)]+\\)'
string = <django.utils.functional.lazy.<locals>.__proxy__ object at 0x10678ca90>
flags = 0

    def search(pattern, string, flags=0):
        """Scan through string looking for a match to the pattern, returning
        a match object, or None if no match was found."""
>       return _compile(pattern, flags).search(string)
E       TypeError: expected string or buffer

../../../../.envs/lingui-next/lib/python3.4/re.py:166: TypeError

[Tomáš Ehrlich]

Added PR ​https://github.com/django/django/pull/3989

[Tomáš Ehrlich]

Also please look at #24221 which is a small optimization of #24133

[Tomáš Ehrlich <tomas.ehrlich@…>]

In 511be35779a98427387d9aa4abacce01dedd7272:

Fixed #24220 - Allowed lazy objects for success_url

[Tim Graham <timograham@…>]

In 5dff3513cc1bb998abe60f52269790268a74220c:

[1.8.x] Fixed #24220 - Allowed lazy objects for success_url

Backport of 511be35779a98427387d9aa4abacce01dedd7272 from master