Less brittle meson install, securer downloads

- This makes installing meson a little less brittle.  It's still not
  ideal I guess.
- Push sha256 checksums on all curl downloads, at least we'll all be
  running the same malware this way.
- Only copy the deltachat wheels to the wheelhouse, that avoids having
  to clean up the unintersting wheels.

python/wheelbuilder/build-wheels.sh

@@ -16,13 +16,10 @@ for PYBIN in /opt/python/*/bin; do
 done
 
 ## Bundle external shared libraries into the wheels
-for whl in wheelhouse/*.whl; do
+for whl in wheelhouse/deltachat*.whl; do
     auditwheel repair "$whl" -w /io/python/wheelhouse/
 done
 
-# Clean up the cffi wheels which end up in the wheelhouse
-rm -rf /io/python/wheelhouse/cffi*.whl
-
 ## Install packages (and test)
 for PYBIN in /opt/python/*/bin/; do
     "${PYBIN}/pip" install deltachat --no-index -f /io/python/wheelhouse