问为什么apt-get升级只返回“以下软件包已被保存在后面:python3-piccopg 2”，而apt更新显示它可以升级(如何)？

EN

sudo apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  python3-psycopg2
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

sudo apt upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

sudo apt update
Reading package lists... Done
Building dependency tree       
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see it.

apt list --upgradable
Listing... Done
python3-psycopg2/stretch-pgdg 2.8.5-1~pgdg90+1 amd64 [upgradable from: 2.7.7-1]
N: There is 1 additional version. Please use the '-a' switch to see it

apt list --upgradable -a
Listing... Done
python3-psycopg2/stretch-pgdg 2.8.5-1~pgdg90+1 amd64 [upgradable from: 2.7.7-1]
python3-psycopg2/stable,now 2.7.7-1 amd64 [installed,upgradable to: 2.8.5-1~pgdg90+1]

apt policy python3-psycopg2
python3-psycopg2:
  Installed: 2.7.7-1
  Candidate: 2.8.5-1~pgdg90+1
  Version table:
     2.8.5-1~pgdg90+1 500
        500 http://apt.postgresql.org/pub/repos/apt stretch-pgdg/main amd64 Packages
 *** 2.7.7-1 500
        500 http://ftp.XX.debian.org/debian buster/main amd64 Packages
        100 /var/lib/dpkg/status

sudo apt install python3-psycopg2
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python3-psycopg2 : Depends: python3 (< 3.6) but 3.7.3-1 is to be installed
E: Unable to correct problems, you have held broken packages.

sudo apt-get --with-new-pkgs upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.