refactor(get-modflow): don't hard-code available os tags (#2426)

Since these are given to change, it seems sufficient to raise (as we do) if a corresponding asset doesn't exist.

flopy/utils/get_modflow.py

@@ -35,15 +35,6 @@
     "modflow6-nightly-build": "modflow6_nightly",
 }
 available_repos = list(renamed_prefix.keys())
-available_ostags = [
-    "linux",
-    "mac",
-    "macarm",
-    "win32",
-    "win64",
-    "win64ext",
-    "win64par",
-]
 max_http_tries = 3
 
 # Check if this is running from flopy
@@ -83,9 +74,7 @@ def get_suffixes(ostag) -> Tuple[str, str]:
     elif "mac" in ostag:
         return "", ".dylib"
     else:
-        raise KeyError(
-            f"unrecognized ostag {ostag!r}; choose one of {available_ostags}"
-        )
+        raise KeyError(f"unrecognized ostag {ostag!r}")
 
 
 def get_request(url, params={}):
@@ -699,7 +688,6 @@ def cli_main():
     )
     parser.add_argument(
         "--ostag",
-        choices=available_ostags,
         help="Operating system tag; default is to automatically choose.",
     )
     parser.add_argument(