Fix sqlalchemy warnings before sqlalchemy 1.4

[jelly]

/usr/lib/python3.8/site-packages/sqlalchemy/orm/relationships.py:1912: SAWarning: Setting backref / back_populates on relationship CVEVersion.version_parent to refer to viewonly relationship CVE.versions will be deprecated in SQLAlchemy 1.4, and will be disallowed in a future release.  viewonly relationships should not be mutated (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.8/site-packages/sqlalchemy/orm/relationships.py:1912: SAWarning: Setting backref / back_populates on relationship CVE.versions to refer to viewonly relationship CVEVersion.version_parent will be deprecated in SQLAlchemy 1.4, and will be disallowed in a future release.  viewonly relationships should not be mutated (this warning may be suppressed after 10 occurrences)

[anthraxx]

some more

/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship CVE.versions to refer to viewonly relationship CVEVersion.version_parent should include sync_backref=False set on the CVE.versions relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship CVEVersion.version_parent to refer to viewonly relationship CVE.versions should include sync_backref=False set on the CVEVersion.version_parent relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship Advisory.versions to refer to viewonly relationship AdvisoryVersion.version_parent should include sync_backref=False set on the Advisory.versions relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship AdvisoryVersion.version_parent to refer to viewonly relationship Advisory.versions should include sync_backref=False set on the AdvisoryVersion.version_parent relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship CVEGroupVersion.version_parent to refer to viewonly relationship CVEGroup.versions should include sync_backref=False set on the CVEGroupVersion.version_parent relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship CVEGroupEntryVersion.version_parent to refer to viewonly relationship CVEGroupEntry.versions should include sync_backref=False set on the CVEGroupEntryVersion.version_parent relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(
/usr/lib/python3.9/site-packages/sqlalchemy/orm/relationships.py:1993: SAWarning: Setting backref / back_populates on relationship CVEGroupPackageVersion.version_parent to refer to viewonly relationship CVEGroupPackage.versions should include sync_backref=False set on the CVEGroupPackageVersion.version_parent relationship.  (this warning may be suppressed after 10 occurrences)
  util.warn_limited(

[jelly]

Related sqlalchemy 1.4 upgrade will most likely blocked by kvesteri/sqlalchemy-continuum#255