update Void Linux installation guide

Remove the requirement to install PyOtherSide manually as it seems to
have been added to Void's package repository some time ago.

docs/INSTALL.md

@@ -279,15 +279,13 @@ pip3 install --user dbus-python
 
 #### Void Linux / xbps
 
-[PyOtherSide](#installing-pyotherside-manually) must be manually installed.
-
 ```sh
 sudo xbps-install -Su qt5-devel qt5-declarative-devel \
                       qt5-quickcontrols2-devel \
                       qt5-svg-devel qt5-graphicaleffects qt5-imageformats \
                       libx11-devel libXScrnSaver-devel alsa-lib-devel \
                       python3-devel python3-pip \
-                      olm-devel \
+                      olm-devel pyotherside \
                       base-devel git cmake \
                       libjpeg-turbo-devel zlib-devel tiff-devel libwebp-devel \
                       libopenjpeg2-devel libmediainfo-devel