Issue #113/#527/#548 add lower bound for pystac requirement and changelog entry

Author: soxofaan

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Start depending on `pystac`, initially for better `load_stac` support ([#133](https://github.com/Open-EO/openeo-python-client/issues/133), [#527](https://github.com/Open-EO/openeo-python-client/issues/527))
+
 ### Changed
 
 ### Removed

setup.py

@@ -73,7 +73,7 @@
         "numpy>=1.17.0",
         "xarray>=0.12.3",
         "pandas>0.20.0",
-        "pystac",
+        "pystac>=1.5.0",  # pystac 1.5.0 is highest version available for lowest Python version we still support (3.7).
         "deprecated>=1.2.12",
         'oschmod>=0.3.12; sys_platform == "win32"',
         "importlib_resources; python_version<'3.9'",