I'm importing data from CSV file into Postgres table using copy from command like this
copy tbl_name(col1, col2, col3) from '/sample.csv' delimiter ',';
the command is executed in a transaction(read-write).
while this command is executing, I'm opening a new SQL session in the new terminal, but in this new session, I'm not able to perform select command. It will be stuck until the transaction is committed in the first session. 
The same issue is happening when in python program I'm copying a file using copy_expert command of Psycopg2, even tho I have created connection_engine with pooling.
Is it possible to prevent Postgres from blocking the er sessions while copy-ing data into the table?
That is impossible, unless you use SELECT ... FOR UPDATE, which tries to put a row lock on rows that are already locked by the COPY.
It is a principle in PostgreSQL that readers don't block writers and vice versa.
copy command blocks other sessions in postgres