ci(win): use PostgreSQL 13 for tests

By latest errors, it seems that Postgres 9.6 is no more supported on
VS2019 image. By documentation, it also seem that Postgres 13 is the
most recent supported database and not available in VS2015 image.
Therefore, drop Python 3.8 test (and likely build).

See https://www.appveyor.com/docs/services-databases/#postgresql

.appveyor/tests.yml

@@ -14,8 +14,6 @@ environment:
     - {PY_VER: "310", PY_ARCH: "64", APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019}
     - {PY_VER: "39", PY_ARCH: "32", APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019}
     - {PY_VER: "39", PY_ARCH: "64", APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019}
-    - {PY_VER: "38", PY_ARCH: "32", APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015}
-    - {PY_VER: "38", PY_ARCH: "64", APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015}
 
   WORKFLOW: tests
 
@@ -34,14 +32,14 @@ environment:
   PERL5LIB: .
 
   # Select according to the service enabled
-  POSTGRES_DIR: C:\Program Files\PostgreSQL\9.6\
+  POSTGRES_DIR: C:\Program Files\PostgreSQL\13\
 
 matrix:
   fast_finish: false
 
 services:
   # Note: if you change this service also change POSTGRES_DIR
-  - postgresql96
+  - postgresql13
 
 cache:
   # Rebuild cache if following file changes