ci: Bump build image on AppVeyor to MSVC 2019 (matplotlib#28869)

* ci: Bump build image on AppVeyor to MSVC 2019

According to the SciPy toolchain roadmap [1], we should be supporting at
minimum MSVC 2019. The AppVeyor image has been held back to MSVC 2017
(probably just forgotten since it didn't complain), which is starting to
cause issues for more modern code.

[1] https://docs.scipy.org/doc/scipy/dev/toolchain.html

* ci: Pin micromamba on AppVeyor to v1

Version 2 appears to be broken WRT installing PyPI packages:
mamba-org/mamba#3467

Author: QuLogic

.appveyor.yml

@@ -17,7 +17,7 @@ skip_commits:
 
 clone_depth: 50
 
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 environment:
 
@@ -45,8 +45,9 @@ cache:
 
 init:
   - ps:
+      # Pinned due to https://github.com/mamba-org/mamba/issues/3467
       Invoke-Webrequest
-      -URI https://micro.mamba.pm/api/micromamba/win-64/latest
+      -URI https://github.com/mamba-org/micromamba-releases/releases/download/1.5.10-0/micromamba-win-64.tar.bz2
       -OutFile C:\projects\micromamba.tar.bz2
   - ps: C:\PROGRA~1\7-Zip\7z.exe x C:\projects\micromamba.tar.bz2 -aoa -oC:\projects\
   - ps: C:\PROGRA~1\7-Zip\7z.exe x C:\projects\micromamba.tar -ttar -aoa -oC:\projects\