feat: allow config files to be processed with YTE (#2269)

This is enabled by adding the top-level key

```yaml
__use_yte__ = true
```

to the config file.

### QC
<!-- Make sure that you can tick the boxes below. -->

* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

docs/snakefiles/configuration.rst

@@ -21,6 +21,9 @@ A configuration is provided as a JSON or YAML file and can be loaded with:
     configfile: "path/to/config.yaml"
 
 The config file can be used to define a dictionary of configuration parameters and their values.
+In case of YAML, the file can optionally be processed with `YTE <https://yte-template-engine.github.io>`_.
+To activate this, you have to add the top-level key ``__use_yte__ = true`` to the YAML file.
+
 In the workflow, the configuration is accessible via the global variable `config`, e.g.
 
 .. code-block:: python

snakemake/io.py

@@ -1694,18 +1694,9 @@ def _load_configfile(configpath_or_obj, filetype="Config"):
             except ValueError:
                 f.seek(0)  # try again
             try:
-                # From https://stackoverflow.com/a/21912744/84349
-                class OrderedLoader(yaml.Loader):
-                    pass
+                import yte
 
-                def construct_mapping(loader, node):
-                    loader.flatten_mapping(node)
-                    return collections.OrderedDict(loader.construct_pairs(node))
-
-                OrderedLoader.add_constructor(
-                    yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG, construct_mapping
-                )
-                return yaml.load(f, Loader=OrderedLoader)
+                return yte.process_yaml(f, require_use_yte=True)
             except yaml.YAMLError:
                 raise WorkflowError(
                     "Config file is not valid JSON or YAML. "

tests/test_config_yte/Snakefile

@@ -0,0 +1,7 @@
+configfile: "config.yaml"
+
+rule a:
+    output:
+        "test.out"
+    shell:
+        "echo {config[test]} > {output}"

tests/test_config_yte/config.yaml

@@ -0,0 +1,3 @@
+__use_yte__: true
+
+test: ?5 + 5

tests/test_config_yte/expected-results/test.out

@@ -0,0 +1 @@
+10

tests/tests.py

@@ -2058,3 +2058,8 @@ def test_localrule():
 @skip_on_windows
 def test_module_wildcard_constraints():
     run(dpath("test_module_wildcard_constraints"))
+
+
+@skip_on_windows
+def test_config_yte():
+    run(dpath("test_config_yte"))