Improve file handling in tnseq_tools

[allrob23]

This pull request addresses an issue with file handling in the tnseq_tools.py script. The current implementation uses open() without properly closing the file, which could potentially lead to resource leaks.

Changes made:

Replace the direct open() call with a with statement to ensure proper file closure.
Improve code readability and adhere to Python best practices for file handling.

Before:

for line in open(metadata_file):
  ....

After:

with open(metadata_file) as f:
  for line in f:
    ....

Motivation:

While Python's garbage collector will eventually close the file when the file object is no longer referenced, it's generally considered good practice to explicitly close files or use a context manager to ensure they're closed promptly and also avoid resource leaks.