Pins astlib to 0.11.8

Deals with a test breaking issue induced by Astlib 0.11.10 under setuptools 59.6.0 which affects Python 3.6 support.

The issue breaks existing tests. Until we migrate to test under Python 3.8 and Python 3.10 this should fix the existing tests.

setup.py

@@ -6,7 +6,15 @@
 __version__ = "1.7.3"
 
 # PyQt 5 has not been added here are. It needs to be installed via apt-get which is a Tigger v1.6.0 requirement.
-requirements = ['astro_kittens', 'numpy', 'scipy', 'astlib', 'astropy', 'future', 'python-casacore']
+requirements = ['astro_kittens', 
+                'numpy', 
+                'scipy', 
+                'astlib<=0.11.10; python_version >="3.8"', 
+                'astlib<=0.11.8; python_version <"3.8"', 
+                'astropy', 
+                'future', 
+                'python-casacore'
+]
 
 scripts = [
     'Tigger/bin/tigger-convert',