Handle requirements.txt files without trailing newlines

[drdavella]

Overview

Correctly generate diffs when adding dependencies to requirements.txt files without trailing newlines

Description

• This was fixed for libcst codemods in general but requirements.txt is a special case

[codecov]

Codecov Report

Merging #140 (a369f68) into main (d0414dd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files          65       66    +1     
  Lines        2727     2732    +5     
=======================================
+ Hits         2627     2632    +5     
  Misses        100      100           

Files	Coverage Δ
src/codemodder/codemodder.py	97.01% <100.00%> (-0.05%)	⬇️
src/codemodder/dependency_manager.py	92.98% <100.00%> (+0.25%)	⬆️
src/codemodder/diff.py	100.00% <100.00%> (ø)