libostree v2020.3

["Colin Walters" <walters verbum org>]

https://github.com/ostreedev/ostree/releases/tag/v2020.3

A quick followup to 2020.2, which introduced support
for [read-only sysroot][1] ended up breaking some of
the Fedora CoreOS tests in [coreos-assembler][2]
which in turn holds back ostree going into FCOS:
https://github.com/coreos/fedora-coreos-tracker/issues/343

Now we've closed that gap and are running more of those
tests as part of our [new CI][3].

[1] https://github.com/ostreedev/ostree/pull/1767/commits/5af403be0cc64df50ad21cef05f3268ead256d6d
[2] https://github.com/coreos/fedora-coreos-tracker/issues/343
[3] https://github.com/coreos/fedora-coreos-tracker/issues/263

```
Colin Walters (3):
      Post-release version bump
      ci: Test kola --upgrades
      main: Also automatically remount rw /sysroot for `ostree pull` etc.

Jonathan Lebon (3):
      ci: migrate to new coreos-ci project
      ci: use `fcosKola` for running kola tests
      Release 2020.3


Git-EVTag-v0-SHA512: 
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```