pip: `[Errno -2] Name or service not known` on amd64 only?

We recently integrated Sphinx-generated documentation into our workflow, but having pushed deb sources to Launchpad, we have the builds failing with:

```
$ python3 -m venv .sphinx/venv
. .sphinx/venv/bin/activate; pip install --upgrade -r .sphinx/requirements.txt
WARNING: The directory '/sbuild-nonexistent/.cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you should use sudo's -H flag.
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7fb746680d10>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/sphinx/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7fb7466817d0>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/sphinx/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7fb746681e90>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/sphinx/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7fb746682c50>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/sphinx/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7fb746683490>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/sphinx/
ERROR: Could not find a version that satisfies the requirement sphinx (from versions: none)
ERROR: No matching distribution found for sphinx
```

This looks like pypi is not available from the builders? Is there an alternative?