improve python caching (#468)

(@dignifiedquire and @hpk42) 
- introduce rust-caching to python test runs 
- skip release and ffi runs, they are check using python bindings
- shuffle files such that ci_scripts/ contains all the ci scripts
- partly parallelize python tox runs

ci_scripts/run-python.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+#
+# Build the Delta Chat C/Rust library typically run in a docker
+# container that contains all library deps but should also work
+# outside if you have the dependencies installed on your system.
+
+set -e -x
+
+# Perform clean build of core and install.
+export TOXWORKDIR=.docker-tox
+
+# install core lib
+
+export PATH=/root/.cargo/bin:$PATH
+cargo build --release -p deltachat_ffi
+# cargo test --all --all-features
+
+# Statically link against libdeltachat.a.
+export DCC_RS_DEV=$(pwd)
+
+# Configure access to a base python and to several python interpreters
+# needed by tox below.
+export PATH=$PATH:/opt/python/cp35-cp35m/bin
+export PYTHONDONTWRITEBYTECODE=1
+pushd /bin
+ln -s /opt/python/cp27-cp27m/bin/python2.7
+ln -s /opt/python/cp36-cp36m/bin/python3.6
+ln -s /opt/python/cp37-cp37m/bin/python3.7
+popd
+
+if [ -n "$TESTS" ]; then
+
+    pushd python
+    # prepare a clean tox run
+    rm -rf tests/__pycache__
+    rm -rf src/deltachat/__pycache__
+    export PYTHONDONTWRITEBYTECODE=1
+
+    # run tox. The circle-ci project env-var-setting DCC_PY_LIVECONFIG 
+    # allows running of "liveconfig" tests but for speed reasons
+    # we run them only for the highest python version we support
+
+    tox --workdir "$TOXWORKDIR" -e py37 
+    unset DCC_PY_LIVECONFIG
+    tox --workdir "$TOXWORKDIR" -p4 -e lint,py35,py36,doc
+    tox --workdir "$TOXWORKDIR" -e auditwheels
+    popd
+fi
+
+
+# if [ -n "$DOCS" ]; then
+#     echo -----------------------
+#     echo generating python docs
+#     echo -----------------------
+#     (cd python && tox --workdir "$TOXWORKDIR" -e doc)
+# fi