Remove tests-based code example from walkthrough. More precise requir…

…ements, back to python 3.9 importlib.resources

doc/requirements.txt

@@ -1,5 +1,6 @@
 lxml=4.9.2
-requests==2.28
+requests==2.28.1
+requests-cache==0.9.7
 pandas==1.5.2
 sphinx==5.3
 ipython==8.7.0

doc/walkthrough.rst

@@ -555,15 +555,8 @@ e.g., to access a cloud storage provider's file system.
 
 Likewise, :func:`.read_sdmx` can be used 
 to load SDMX messages stored in local files or 
-remote files using FSSPEC:
-
-.. ipython:: python
-
-    # Use an example ('specimen') file from the pandaSDMX test suite
-    from pandasdmx.tests.data import specimen
-    # …with time-series exchange rate data from the EU Central Bank
-    with specimen('ECB_EXR/ng-ts.xml') as f:
-        sdmx.read_sdmx(f)
+remote files using 
+`FSSPEC <https://filesystem-spec.readthedocs.io/en/latest/>`__.
 
 
 Handle errors

pandasdmx/source/__init__.py

@@ -207,8 +207,7 @@ def list_sources():
 
 def load_package_sources():
     """Discover all sources listed in ``sources.json``."""
-    # py3.9+: with resources.files("pandasdmx").joinpath("sources.json").open("rb") as f:
-    with resources.open_binary("pandasdmx", "sources.json") as f:
+    with resources.files("pandasdmx").joinpath("sources.json").open("rb") as f:
         for info in json.load(f):
             add_source(info)