apt.Cache.update with alternative sources.list

[Ole Streicher <olebole@debian.org>]

Hi,

I need some access (as normal user) to an apt cache with an alternative
sources.list (those in /etc/blends/ installed by blends-dev), but I have
problems to find out how to use it.

I first tried to just specify the alternative source file:

```
>>> import apt
>>> c = apt.Cache()
>>> c.update('sources_list='/etc/blends/sources.list.testing')
LockFailedException: Failed to lock /var/lib/apt/lists/lock
```

Then I tried to use an alternative root directory:

```
>>> import apt
>>> c = apt.Cache(rootdir='/tmp/myapttmp')
>>> c.update('sources_list='/etc/blends/sources.list.testing')
FetchFailedException: W:GPG error: http://ftp.debian.org/debian testing InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010, E:The repository 'http://ftp.debian.org/debian testing InRelease' is not signed.
```

To which place in the new root directory do I need to copy the keyring?
I tried <<rootdir>>/usr/share/keyrings/ but this didn't work.

And/or how can I disable authentication (--allow-unauthenticated,
resp. APT::Get::AllowUnauthenticated)?

Version of python3_apt is 1.4.0~beta3+b1

Best regards

Ole