build(python): change extra names to comply with PEP 685 (#446)

Since pip>=23.3.0, non-normalised extra names are ignored, so
`snakemake_reports` is changed to `snakemake-reports`. The former name
is kept for backward compatibility with older versions of pip.

setup.py

@@ -41,7 +41,7 @@
         "tabulate<0.9",
         "pulp>=2.7.0,<2.8.0",
     ],
-    "snakemake_reports": [
+    "snakemake-reports": [
         "snakemake==6.15.5 ; python_version<'3.7'",
         "snakemake==7.32.4 ; python_version>='3.7'",
         "pygraphviz<1.8",
@@ -50,6 +50,9 @@
     ],
 }
 
+# backwards compatibility with extras before PEP 685
+extras_require["snakemake_reports"] = extras_require["snakemake-reports"]
+
 extras_require["all"] = []
 for key, reqs in extras_require.items():
     if ":" == key[0]: