Replace Python 3.7 with 3.11 on Appveyor

This changes the tests labelled as "Test alternative Python version"
from Python 3.7 to 3.11.

A move to 3.8 would sound logical, but... if I'm seeing correctly, the
only difference between Ubu20P37a & Ubu20P37b versus Ubu20core and
Ubu20a1 is that the former had "PY: 3.7", while the latter did not set
PY at all. And if I'm reading the logs of the latest Appveyor run
correctly, Ubu20core & Ubu20a1 run on Python 3.8.10. I cound not find
confirmation that this is the default, but it seems to be.

An alternative would be to set PY for Ubu20core & Ubu20a1 to the
newest supported (3.11) and use the other two to test oldest
supported (3.8). But maybe appveyor tends to stay on the lowest
non-EOL version when PY is not specified, in which case the current
setup would be preferred (just a guess - couldn't find confirmation)?

Related to datalad#7400

.appveyor.yml

@@ -168,9 +168,9 @@ environment:
       CODECOV_BINARY: https://uploader.codecov.io/latest/macos/codecov
 
     # Test alternative Python versions
-    - ID: Ubu20P37a
+    - ID: Ubu20P311a
       # ~35min
-      PY: 3.7
+      PY: 3.11
       DTS: >
           datalad.cli
           datalad.core
@@ -183,9 +183,9 @@ environment:
       CODECOV_BINARY: https://uploader.codecov.io/latest/linux/codecov
       # system git-annex is way too old, use better one
       INSTALL_GITANNEX: git-annex -m deb-url --url https://datasets.datalad.org/datalad/packages/neurodebian/git-annex_8.20210903-1_amd64.deb
-    - ID: Ubu20P37b
+    - ID: Ubu20P311b
       # ~25min
-      PY: 3.7
+      PY: 3.11
       DTS: >
           datalad.downloaders
           datalad.interface