Add python-snappy & lz4 requirements to CI

These should already be satisfied when installed. Though this makes it
clearer that these are the required versions (particularly for any users
poking around trying to understand things).

continuous_integration/environment-3.9.yaml

@@ -19,7 +19,7 @@ dependencies:
   - ipywidgets
   - jinja2
   - locket >=1.0
-  - lz4  # Only tested here
+  - lz4 >=0.23.1  # Only tested here
   - msgpack-python
   - netcdf4
   - paramiko
@@ -32,7 +32,7 @@ dependencies:
   - pytest-repeat
   - pytest-rerunfailures
   - pytest-timeout
-  - python-snappy  # Only tested here
+  - python-snappy >=0.5.3  # Only tested here
   - pytorch  # Only tested here
   - requests
   - s3fs