The compatibility issue in meson v0.53 has been fixed in v0.53.1, which
is already available through pip, so let's remove the pin for meson
introduced before.
Reverts: 514793658c
Latest meson doesn't work with older python 3.5, which is present on
Ubuntu 16.04. Let's pin in to the latest working version (0.52.1) until
we properly bump all necessary Ubuntu images to 18.04.
See: https://github.com/mesonbuild/meson/issues/6427