Does CREATE TABLE AS lock the table?
No it does not create a read or write lock on films table as this creates ACCESS SHARE LOCK;

to test this  open two sessions

in session one  run this command

  begin;
  create table films2 as 
     table films;


then in the second session run this command

Select * from films;
Update films set something = 0


If you need to lock films table you have to issue a Lock like so

begin ;
  Lock table films in exclusive mode;
  create table films2 as 
     table films;
commit;


this will block the table from updates but not reads...

NOTE  Access Share Locks block any transaction that try to acquire an ACCESS EXCLUSIVE locks such as DROP Films etc...