(core) freshen core README; support python3 in grist-core docker image

Summary:
This updates the grist-core README to list specific features of Grist,
to make it easier for a casual visitor to get a sense of its scope. Adds links
to some new resources (reviews, templates, grist v airtable post) that could
also help. Adds python3 to docker image so that templates work without fuss.

Test Plan: existing tests should pass

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3204

buildtools/prepare_python.sh

@@ -2,10 +2,14 @@
 
 set -e
 
-if [ ! -e venv ]; then
-  virtualenv -ppython2.7 venv
+echo "Use Python3 if available and recent enough, otherwise Python2"
+if python3 -c 'import sys; assert sys.version_info >= (3,9)' 2> /dev/null; then
+  # Default to python3 if recent enough.
+  buildtools/prepare_python3.sh
+  # Make sure python2 isn't around.
+  rm -rf venv
+else
+  buildtools/prepare_python2.sh
+  # Make sure python3 isn't around.
+  rm -rf sandbox_venv3
 fi
-
-. venv/bin/activate
-
-pip install --no-deps -r sandbox/requirements.txt

buildtools/prepare_python2.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -e
+
+echo "Making Python2 sandbox"
+if [ ! -e venv ]; then
+  virtualenv -ppython2.7 venv
+fi
+
+. venv/bin/activate
+
+echo "Updating Python2 packages"
+pip install --no-deps -r sandbox/requirements.txt
+echo "Python2 packages ready in venv"

buildtools/prepare_python3.sh

@@ -2,10 +2,11 @@
 
 set -e
 
+echo "Making Python3 sandbox"
 if [ ! -e sandbox_venv3 ]; then
-  virtualenv -ppython3 sandbox_venv3
+  python3 -m venv sandbox_venv3
 fi
 
-. sandbox_venv3/bin/activate
-
-pip install --no-deps -r sandbox/requirements3.txt
+echo "Updating Python3 packages"
+sandbox_venv3/bin/pip install --no-deps -r sandbox/requirements3.txt
+echo "Python3 packages ready in sandbox_venv3"