Python 3.8 with libs needed for Mercurial

As can be seen in the changeset descriptions, complications arose due to upstream GitLab patching out of readline (in favor of editline) and of sqlite (to avoid a dependency on a lib not built by Omnibus). In both cases, we chose to rely on the underlying Linux distribution, as it should be a long time before that becomes more problematic than building libsqlite3 or patching CPython.

Closes #4 (closed)