muradm,

Thank you so much for your help! That worked like a charm! Just needed to
adjust the second argument of with-output-to-file so that it takes a lambda
that returns (newline).
Here is the resulting package definition that successfully builds.

(define-public python-verspec
  (package
    (name "python-verspec")
    (version "0.1.0")
    (source
      (origin
        (method url-fetch)
        (uri (pypi-uri "verspec" version))
        (sha256
          (base32
            "07n06wv85fm4vl1ird2mja0823js3x322wgs9gdnq1djjyk4ql64"))
        (modules '((guix build utils)))
        (snippet
        '(with-directory-excursion "test"
           (with-output-to-file "__init__.py"
             (lambda () (newline)))))))
    (build-system python-build-system)
    (native-inputs
      `(("python-coverage" ,python-coverage)
        ("python-flake8" ,python-flake8)
        ("python-mypy" ,python-mypy)
        ("python-pretend" ,python-pretend)
        ("python-pytest" ,python-pytest)))
    (home-page
      "https://github.com/jimporter/verspec")
    (synopsis "Flexible version handling")
    (description "Flexible version handling")
    (license '(license:asl2.0 license:bsd-2))))

Regards,
Antwane