Further minor fixes for Python 3.10 support

Adds support for Python v3.10 and Tensorflow 2.15

Minor release to disallow dependence on TF 2.8.0.

Fixes a major issue with deadlocking introduced in the TFv2 versions 1.7.0 and above. This release has been fully tested using DDFacet in DFT mode.

Minor release to fix deprecation warnings inside montblanc

Major release to upgrade Montblanc to use the compatibility layers of more recent Tensorflow 2.7, which is installable on Ubuntu 20.04 systems
(ships Python 3.8 by default)

Improved support for pep 517 source distribution builds

This is the last version of Montblanc that requires user coordinate inputs as lm (cosines on the unit sphere). As is in use with ratt-ru/cubical@master