Use context manager for reading Excel file

[phackstock]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

Closes #817.
Use a context manager to ensure that an Excel file is closed after reading.

FYI @adrivinca

[danielhuppmann]

Looks good, thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.8%. Comparing base (0762055) to head (e503e65).

Files	Patch %	Lines
pyam/utils.py	90.9%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #818   +/-   ##
=====================================
  Coverage   94.8%   94.8%           
=====================================
  Files         64      64           
  Lines       6089    6089           
=====================================
  Hits        5775    5775           
  Misses       314     314           

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[phackstock]

Pinned ixmp4==0.6.0 after a test broke when using ixmp4 version 0.7.0.