Update to upstream version '0.0.55'
with Debian dir 757a1994777f26411a82369bb5efa044e815d8eb

The current postgresql model refuses NULL values.

Update to upstream version '0.0.54'
with Debian dir bfe97b183746e3dc69c02c2e023a1b921944ae16

Update to upstream version '0.0.53'
with Debian dir 9ae06f89508d79e2b8bd4b6010c5ee659f186a06

Related to P589

This allows for example to type:

  curl -s https://archive.softwareheritage.org/browse/content/sha1_git:64582b78792cd6c2d67d35da5a11bb80886a6409/raw/ | swh identify
  swh:1:cnt:64582b78792cd6c2d67d35da5a11bb80886a6409	-

Related to T2188

Update to upstream version '0.0.52'
with Debian dir 95c5716132b2b4cd84b3f5a5dd75bb25bf9f3654

Add support to automatically compute identifier in the following object models:
Directory, Release, Revision, Snapshot.

If the identifier is not provided as parameter, it will be computed when the model
is initialized.

Allows us to call tests on things other than python3, with the same settings.

they will remain for internal use only, so should not be mentioned in this
public spec

this reverts the documentation part of 67fade5f

Update to upstream version '0.0.51'
with Debian dir 0807fecfe56a10d6de3e5d0c3819238a3053bb94

Update to upstream version '0.0.50'
with Debian dir dd9155d8adbd0e75c08056bb9e558c368733b778

currently provides mainly 2 generators:
- gen_origins()
- gen_contents()

we do not really need them to be mutable, plus we gain their instances now
being hashable, so we can add them in set() for example.