Use a variable to set the default request timeout (#418)

Using a variable in `pooch.downloaders` allows users to potentially
increase it if they require and allows us to update it easily if
required as well. Set it to 30s instead of 5s to allow for slow
connections.

pooch/downloaders.py

@@ -26,6 +26,12 @@
     paramiko = None
 
 
+# Set the default timeout in seconds so it can be configured in a pinch for the
+# methods that don't or can't expose a way set it at runtime.
+# See https://github.com/fatiando/pooch/issues/409
+DEFAULT_TIMEOUT = 30
+
+
 def choose_downloader(url, progressbar=False):
     """
     Choose the appropriate downloader for the given URL based on the protocol.
@@ -197,13 +203,13 @@ def __call__(
         import requests  # pylint: disable=C0415
 
         if check_only:
-            timeout = self.kwargs.get("timeout", 5)
+            timeout = self.kwargs.get("timeout", DEFAULT_TIMEOUT)
             response = requests.head(url, timeout=timeout, allow_redirects=True)
             available = bool(response.status_code == 200)
             return available
 
         kwargs = self.kwargs.copy()
-        timeout = kwargs.pop("timeout", 5)
+        timeout = kwargs.pop("timeout", DEFAULT_TIMEOUT)
         kwargs.setdefault("stream", True)
         ispath = not hasattr(output_file, "write")
         if ispath:
@@ -640,7 +646,7 @@ def doi_to_url(doi):
     import requests  # pylint: disable=C0415
 
     # Use doi.org to resolve the DOI to the repository website.
-    response = requests.get(f"https://doi.org/{doi}", timeout=5)
+    response = requests.get(f"https://doi.org/{doi}", timeout=DEFAULT_TIMEOUT)
     url = response.url
     if 400 <= response.status_code < 600:
         raise ValueError(
@@ -800,7 +806,7 @@ def api_response(self):
             article_id = self.archive_url.split("/")[-1]
             self._api_response = requests.get(
                 f"{self.base_api_url}/{article_id}",
-                timeout=5,
+                timeout=DEFAULT_TIMEOUT,
             ).json()
 
         return self._api_response
@@ -966,7 +972,7 @@ def api_response(self):
             # Use the figshare API to find the article ID from the DOI
             article = requests.get(
                 f"https://api.figshare.com/v2/articles?doi={self.doi}",
-                timeout=5,
+                timeout=DEFAULT_TIMEOUT,
             ).json()[0]
             article_id = article["id"]
             # Parse desired version from the doi
@@ -993,7 +999,7 @@ def api_response(self):
                     f"{article_id}/versions/{version}"
                 )
             # Make the request and return the files in the figshare repository
-            response = requests.get(api_url, timeout=5)
+            response = requests.get(api_url, timeout=DEFAULT_TIMEOUT)
             response.raise_for_status()
             self._api_response = response.json()["files"]
 
@@ -1087,7 +1093,7 @@ def _get_api_response(cls, doi, archive_url):
         response = requests.get(
             f"{parsed['protocol']}://{parsed['netloc']}/api/datasets/"
             f":persistentId?persistentId=doi:{doi}",
-            timeout=5,
+            timeout=DEFAULT_TIMEOUT,
         )
         return response