Pin pystac to 1.4.0 for now

newer version of raster extension breaks tests.

We'll need to work around overly strict version
checking in pystac library or just not use extension
classes and look for data in dicts directly, it's
not realistic to expect all data and all software
to always use compatible versions in any given
installation.

stac-utils/pystac#448

tests/test-env-py38.yml

@@ -14,7 +14,7 @@ dependencies:
   - jinja2
   - numpy
   - pandas
-  - pystac >=1.1.0
+  - pystac ==1.4.0
   - toolz
   - xarray ~=0.20.1
   - odc-geo >=0.2.0