I have a large dump file created by mongodump utility, for example "test.dump". I want get one exact collection from this dump, and manually read it into memory for further processing as valid BSON documents. I cannot load full dump in memory due to it's size. 
I do not need physically restore anything to mongo instances! I basically even have none of them up and running. So mongorestore utility could be a solution only if can help me to read my collection from a dump file to memory.
I'm using Python 3 and pymongo, but can import another third-party libs if necessary or launch any CLI utilities with stdout results.
I am unfamiliar with any off-the-shelf tools that would extract a collection out of a dump file. That said:

AWS offers x1e.32xlarge instance type with almost 4 TB of memory. How big is your dump exactly?
Surely the easiest solution is to just load the dump into a MongoDB deployment (which doesn't need much memory or other resources, if you are going to dump one collection back). Hardware is very cheap these days.
The BSON format is not that complicated. I expect you'd need to write the tooling for this yourself but if the dump is in fact valid BSON you can manually traverse it using BSON reading code that is part of every MongoDB driver.
Read mongodump manually