python3Packages.dateparser: patch flaky test

The dateparser module has a test that works with the current day of the
month and uses it in June 2020. This breaks everytime we build the
package on the 31st of a month, because June only has 30 days.

```
❯ cal 6 2020
      Juni 2020
Mo Di Mi Do Fr Sa So
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30
```

scrapinghub/dateparser#1053

pkgs/development/python-modules/dateparser/default.nix

@@ -29,6 +29,13 @@ buildPythonPackage rec {
     sha256 = "sha256-bDup3q93Zq+pvwsy/lQy2byOMjG6C/+7813hWQMbZRU=";
   };
 
+  postPatch = ''
+    # https://github.com/scrapinghub/dateparser/issues/1053
+    substituteInPlace tests/test_search.py --replace \
+      "('June 2020', datetime.datetime(2020, 6, datetime.datetime.utcnow().day, 0, 0))," \
+      "('June 2020', datetime.datetime(2020, 6, min(30, datetime.datetime.utcnow().day), 0, 0)),"
+  '';
+
   propagatedBuildInputs = [
     # install_requires
     python-dateutil pytz regex tzlocal