Bump marshmallow from 2.12.2 to 2.15.1

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 2.12.2 to 2.15.1.
- [Release notes](https://github.com/marshmallow-code/marshmallow/releases)
- [Changelog](https://github.com/marshmallow-code/marshmallow/blob/dev/CHANGELOG.rst)
- [Commits](marshmallow-code/marshmallow@2.12.2...2.15.1)

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -18,7 +18,6 @@
 # In applying this license, CERN does not
 # waive the privileges and immunities granted to it by virtue of its status
 # as an Intergovernmental Organization or submit itself to any jurisdiction.
-
 alembic==0.8.10           # via flask-alembic
 amqp==2.2.1               # via invenio-queues, kombu
 angular-gettext-babel==0.3  # via invenio-search-ui
@@ -84,7 +83,6 @@ invenio-marc21==1.0.0a5
 invenio-oaiserver==1.0.0a13
 invenio-oauth2server==1.0.0a14
 invenio-oauthclient==1.0.0b2
-invenio-pidrelations==v1.0.0a4
 invenio-pidstore==1.0.0b2
 invenio-query-parser==0.6.0
 invenio-queues==1.0.0a1   # via invenio-stats
@@ -108,7 +106,7 @@ kombu==4.1.0              # via celery, invenio-queues
 lxml==3.6.4               # via datacite, dcxml, dojson, invenio-oaiserver
 mako==1.0.6               # via alembic
 markupsafe==0.23          # via jinja2, mako
-marshmallow==2.12.2       # via invenio-oaiserver, invenio-records-rest, webargs
+marshmallow==2.15.1       # via invenio-oaiserver, invenio-records-rest, webargs
 maxminddb-geolite2==2017.404  # via invenio-accounts, invenio-stats
 maxminddb==1.3.0          # via maxminddb-geolite2
 msgpack-python==0.4.8     # via invenio-celery