[QGIS-Developer] "atlasprint" plugin, version 3.1.0, is listed as available - download throws HTTP 404

[Sebastian M. Ernst]

Just a small find: "atlasprint" version 3.1.0 is listed as available for
QGIS >= 3.0. Trying to grab it via its download URL throws HTTP 404.

Verify with:
wget --user-agent="QGIS"
https://plugins.qgis.org/plugins/atlasprint/version/v3.1.0/download/

Excerpt from `plugins.xml`:

```
<pyqgis_plugin name="atlasprint" version="v3.1.0" plugin_id="1893">
        <description><![CDATA[Add atlas capabilities for getPrint
request in WMS for QGIS Server]]></description>
        <about><![CDATA[Add atlas capability for getPrint request]]></about>
        <version>v3.1.0</version>
        <trusted>True</trusted>
        <qgis_minimum_version>3.0.0</qgis_minimum_version>
        <qgis_maximum_version>3.99.0</qgis_maximum_version>

<homepage><![CDATA[https://github.com/3liz/qgis-atlasprint]]></homepage>
        <file_name>atlasprint.v3.1.0.zip</file_name>
        <icon>/media/packages/2020/icon_3DER6Id.png</icon>
        <author_name><![CDATA[3Liz]]></author_name>

<download_url>https://plugins.qgis.org/plugins/atlasprint/version/v3.1.0/download/</download_url>
        <uploaded_by><![CDATA[Gustry]]></uploaded_by>
        <create_date>2020-04-28T05:18:01.270791</create_date>
        <update_date>2020-04-28T05:18:01.270791</update_date>
        <experimental>False</experimental>
        <deprecated>False</deprecated>

<tracker><![CDATA[https://github.com/3liz/qgis-atlasprint/issues]]></tracker>

<repository><![CDATA[https://github.com/3liz/qgis-atlasprint]]></repository>
        <tags><![CDATA[atlas,server,print]]></tags>
        <downloads>2763</downloads>
        <average_vote>2.9995000833194467</average_vote>
        <rating_votes>6</rating_votes>
        <external_dependencies></external_dependencies>
        <server>True</server>
    </pyqgis_plugin>
```