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?
This is not a general phenomenon.  There is more going on here than you are telling us.
Maybe the COPY is happening inside the same transaction as something else which acquires a strong lock (like TRUNCATE) on the same table that is being SELECTed from.  Or maybe your SELECT is invoking some user-defined-function (perhaps directly, perhaps through a trigger or something) which is acquiring a stronger lock than SELECT usually requires.
In the absence of special conditions such as those, I have no problem running COPY and SELECT at the same time.
copy command blocks other sessions in postgres