Replace upload scripts

Replace the existing upload scripts with
a python wrapper for the release API, and
a more compact shell script calling the wrapper.

The latest artifacts are placed in "continuous",
and also placed under the "continuous-rotating"
release, that contains a rotating history of
recent builds.

Note on semantics: the meaning of "rotate" in the
old upload script referred to existing assets being
replaced by the new ones (if they have the same name).
It now refers to the oldest assets being rotated out
of the list.

.travis.yml

@@ -1,6 +1,6 @@
 language: generic
 os: linux
-dist: trusty
+dist: xenial
 compiler: g++
 sudo: required
 services: docker
@@ -9,27 +9,37 @@ git:
   quiet: true
   depth: 1
 
+addons:
+  apt:
+    packages:
+      python3
+      python3-pip
+
+install:
+  - pip3 install requests
+
 before_script:
-  - python scripts/outputstreams_set_blocking.py
+  - python --version
+  - python3 --version
   - git clone https://github.com/mypaint/mypaint.git
   - git clone https://github.com/mypaint/libmypaint.git
   - git clone https://github.com/mypaint/mypaint-brushes.git
-  - 'if [ -e travis.cancel ]; then
-        exit 0;
-    fi'
-  - travis_wait 20 # 20 minutes should be plenty to pull the docker image
+  - # 20 minutes should be plenty to pull the docker image
+  - travis_wait 20
     sudo docker pull mypaint/appimage-base:1.0.0
-  - sudo docker run -t -v $(pwd):/sources -e "GIT_BRANCH=master"
-    mypaint/appimage-base:1.0.0 bash /sources/scripts/mkappimage.sh
+
+script:
+  - sudo docker run -t -v $(pwd):/sources mypaint/appimage-base:1.0.0
+    bash /sources/scripts/mkappimage.sh
 
 after_success:
     - cd $TRAVIS_BUILD_DIR
-    - ls -lh out/* # Assuming you have some files in out/ that you would like to upload
-    - wget -c https://github.com/aferrero2707/uploadtool/raw/master/upload_rotate.sh
-    - bash  ./upload_rotate.sh "continuous" out/* >& /dev/null
+    - ls -lh out/*
+    - bash ./scripts/upload.sh out/*
 
 branches:
   except:
     - # Do not build tags that we create when we upload to GitHub Releases
     - /^(?i:continuous)$/
+    - /^(?i:continuous-rotating)$/