stack/slapos.cfg: Version up beautifulsoup4=4.12.2:whl

Except for python2: keep beautifulsoup4=4.8.2.

beautifulsoup4=4.12.2 requires flit as build backend.

stack/slapos.cfg

@@ -162,7 +162,7 @@ backports.functools-lru-cache = 1.6.1:whl
 backports.lzma = 0.0.14
 backports.shutil-get-terminal-size = 1.0.0
 bcrypt = 3.1.4
-beautifulsoup4 = 4.8.2
+beautifulsoup4 = 4.12.2:whl
 bleach = 5.0.1
 blinker = 1.6.3:whl
 CacheControl = 0.12.6:whl
@@ -381,6 +381,7 @@ Werkzeug = 2.0.2
 
 [versions:python2]
 attrs = 18.2.0
+beautifulsoup4 = 4.8.2
 certifi = 2020.4.5.1
 charset-normalizer = 2.0.12
 click = 6.7