Fix convert when requirements is a path to a requirements file

[maximlt]

Fixes #8346

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.42%. Comparing base (d605c2b) to head (843ae38).

Files with missing lines	Patch %	Lines
panel/io/convert.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8366      +/-   ##
==========================================
- Coverage   86.08%   85.42%   -0.66%     
==========================================
  Files         349      349              
  Lines       54709    54709              
==========================================
- Hits        47096    46737     -359     
- Misses       7613     7972     +359     

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