Fix min_deps_check; revert to support numpy=1.14 and pandas=0.24

Fixes the issue noticed in:
pydata#4175 (comment)

ci/min_deps_check.py

@@ -141,21 +141,22 @@ def process_pkg(
     policy_months = POLICY_MONTHS.get(pkg, POLICY_MONTHS_DEFAULT)
     policy_published = datetime.now() - timedelta(days=policy_months * 30)
 
-    policy_major = req_major
-    policy_minor = req_minor
-    policy_published_actual = req_published
-    for (major, minor), published in reversed(sorted(versions.items())):
-        if published < policy_published:
-            break
-        policy_major = major
-        policy_minor = minor
-        policy_published_actual = published
+    policy_versions = {
+        version: date for version, date in versions.items() if date < policy_published
+    }
+    version_and_date = lambda version: (version, policy_versions[version])
+    policy_major, policy_minor = max(policy_versions, key=version_and_date)
+    policy_published_actual = versions[policy_major, policy_minor]
 
     if (req_major, req_minor) < (policy_major, policy_minor):
         status = "<"
     elif (req_major, req_minor) > (policy_major, policy_minor):
         status = "> (!)"
-        error("Package is too new: " + pkg)
+        error(
+            f"Package is too new: {pkg}={req_major}.{req_minor} was "
+            f"published on {versions[req_major, req_minor]:%Y-%m-%d} "
+            f"(within the past {policy_months} months)"
+        )
     else:
         status = "="
 

ci/requirements/py36-bare-minimum.yml

@@ -8,6 +8,6 @@ dependencies:
   - pytest
   - pytest-cov
   - pytest-env
-  - numpy=1.15
-  - pandas=0.25
+  - numpy=1.14
+  - pandas=0.24
   - setuptools=41.2

ci/requirements/py36-min-all-deps.yml

@@ -31,8 +31,8 @@ dependencies:
   - nc-time-axis=1.2
   - netcdf4=1.4
   - numba=0.44
-  - numpy=1.15
-  - pandas=0.25
+  - numpy=1.14
+  - pandas=0.24
   # - pint  # See py36-min-nep18.yml
   - pip
   - pseudonetcdf=3.0