Missing `seekable()` method in Files

[Carson Gee]

Neither in django/core/files/utils.py or django/core/files/base.py::File is the seekable method implemented. In python 3 this results in issues when attempting to use python standard interfaces such as bz2 or tarfile and results in AttributeError: 'File' object has no attribute 'seekable' errors when using File class objects.

I'm not sure the best way to patch this though. The seekable property isn't available in python2, but I think it can be duplicated in 2, by checking if it has seek, tell, and truncate based on ​https://docs.python.org/3/library/io.html#io.IOBase.seekable. I'm happy to write the patch, but it would be worth knowing the preference for how to fix it.

[Tim Graham]

An acceptable option might be to implement the method only on Python 3 if it's not part of the standard interface on Python 2.

[Carson Gee]

I have now submitted a patch with tests: ​https://github.com/django/django/pull/4842

[Moritz Sichert]

The code looks good, but it should at least be documented in docs/ref/files/file.txt.

[Tim Graham <timograham@…>]

In e93e0c03:

Fixed #24963 -- Added File.seekable() on Python 3.