fix(download.py): accommodate missing gh api response item (#162)

same fix as modflowpy/flopy#2320

modflow_devtools/download.py

@@ -156,8 +156,8 @@ def get_release(repo, tag="latest", retries=3, verbose=False) -> dict:
         try:
             with urllib.request.urlopen(request, timeout=10) as resp:
                 result = resp.read()
-                remaining = int(resp.headers["x-ratelimit-remaining"])
-                if remaining <= 10:
+                remaining = resp.headers.get("x-ratelimit-remaining", None)
+                if remaining and int(remaining) <= 10:
                     warn(
                         f"Only {remaining} GitHub API requests remaining "
                         "before rate-limiting"