windows binaries: bump python version (3.7.9->3.8.7)

Had to also bump ubuntu version 18.04->20.04,
as was getting errors running the self-compiled pyinstaller otherwise (weird...):

```
from .utils.git import get_repo_revision
ModuleNotFoundError: No module named 'PyInstaller.utils'
```
(similar to https://github.com/pyinstaller/pyinstaller/issues/4403 )

-----

Note re appimage: when trying to compile python 3.8.x on ubuntu 16.04, I am getting:

./Modules/posixmodule.c: In function ‘os_copy_file_range_impl’:
./Modules/posixmodule.c:10351:15: error: implicit declaration of function ‘copy_file_range’ [-Werror=implicit-function-declaration]
         ret = copy_file_range(src, p_offset_src, dst, p_offset_dst, count, flags);

This is because ubuntu 16.04 has too old glibc.

contrib/build-linux/appimage/Dockerfile

@@ -1,6 +1,10 @@
+# Note: we deliberately use an old Ubuntu LTS as base image.
+# from https://docs.appimage.org/introduction/concepts.html :
+# "[AppImages] should be built on the oldest possible system, allowing them to run on newer system[s]"
 FROM ubuntu:16.04@sha256:a4fc0c40360ff2224db3a483e5d80e9164fe3fdce2a8439d2686270643974632
 
 ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
+ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update -q && \
     apt-get install -qy \

contrib/build-linux/appimage/build.sh

@@ -13,6 +13,8 @@ CACHEDIR="$CONTRIB_APPIMAGE/.cache/appimage"
 export GCC_STRIP_BINARIES="1"
 
 # pinned versions
+# note: compiling python 3.8.x requires at least glibc 2.27,
+#       which is first available on ubuntu 18.04
 PYTHON_VERSION=3.7.9
 PKG2APPIMAGE_COMMIT="eb8f3acdd9f11ab19b78f5cb15daa772367daf15"
 SQUASHFSKIT_COMMIT="ae0d656efa2d0df2fcac795b6823b44462f19386"