circleci: install libleveldb-dev _always_.

Python package `plyvel` doesn't have pre-built wheels for Python 3.8
yet, so the CI machines need to build the wheel themselves. They
need LevelDB header files for that.

This change installs the headers system-wide.

SQUASHED:

CircleCI YAML can't handle complex map merging via anchors. So,
do the blunt thing, and install the dependency on _all_ CI runs,
even ones that don't need it.

.circleci/config.yml

@@ -6,6 +6,10 @@ version: 2.0
 common: &common
   working_directory: ~/repo
   steps:
+    # TODO: remove this step when `plyvel` gets a Python 3.8 wheel
+    - run:
+        name: install LevelDB headers dependency system-wide
+        command: sudo apt-get install libleveldb-dev
     - checkout
     - run:
         name: checkout fixtures submodule