Rework requests-cache

We should only be running this once per invocation, not for every
row we check. This should be more efficient, but it means that we
don't cache responses when running via pytest, which is actually
probably a good thing.

csv_metadata_quality/app.py

@@ -1,11 +1,14 @@
 # SPDX-License-Identifier: GPL-3.0-only
 
 import argparse
+import os
 import re
 import signal
 import sys
+from datetime import timedelta
 
 import pandas as pd
+import requests_cache
 from colorama import Fore
 
 import csv_metadata_quality.check as check
@@ -84,6 +87,19 @@ def run(argv):
     else:
         exclude = list()
 
+    # enable transparent request cache with thirty days expiry
+    expire_after = timedelta(days=30)
+    # Allow overriding the location of the requests cache, just in case we are
+    # running in an environment where we can't write to the current working di-
+    # rectory (for example from csv-metadata-quality-web).
+    REQUESTS_CACHE_DIR = os.environ.get("REQUESTS_CACHE_DIR", ".")
+    requests_cache.install_cache(
+        f"{REQUESTS_CACHE_DIR}/agrovoc-response-cache", expire_after=expire_after
+    )
+
+    # prune old cache entries
+    requests_cache.delete()
+
     for column in df.columns:
         if column in exclude:
             print(f"{Fore.YELLOW}Skipping {Fore.RESET}{column}")