Specify encoding="utf-8" in all open functions for future proofing against Python 3.15+

[matthewfeickert]

As @henryiii noted in his PyHEP 2022 talk "What's new in Python 3.11"

3.15 will make unicode default! Always specify an encoding with any open function. You can use "native" encoding in 3.11+ (but, why?).

open(..., encoding="utf-8")
Path(...).read_txt(encoding="utf-8")

which means that encoding="utf-8" should be added to all open() and click.open_file() calls in pyhf.

As of 511d8a9 this includes

git grep --name-only "open(\|open_file("
docs/development.rst
docs/examples/notebooks/ImpactPlot.ipynb
docs/examples/notebooks/binderexample/StatisticalAnalysis.ipynb
docs/examples/notebooks/multiBinPois.ipynb
docs/examples/notebooks/multichannel-coupled-histo.ipynb
docs/examples/notebooks/pullplot.ipynb
docs/likelihood.rst
src/pyhf/cli/infer.py
src/pyhf/cli/patchset.py
src/pyhf/cli/rootio.py
src/pyhf/cli/spec.py
src/pyhf/contrib/utils.py
src/pyhf/readxml.py
src/pyhf/schema/loader.py
src/pyhf/writexml.py
tests/conftest.py
tests/contrib/test_contrib_utils.py
tests/contrib/test_viz.py
tests/test_export.py
tests/test_modifiers.py
tests/test_patchset.py
tests/test_pdf.py
tests/test_schema.py
tests/test_scripts.py
tests/test_validation.py
tests/test_workspace.py
validation/makedata.py
validation/manualonoff_roofit/onoff.py
validation/multichan_coupledhistosys_histfactory/makedata.py
validation/multichan_coupledoverall_histfactory/makedata.py
validation/multichannel_histfactory/makedata.py
validation/run_toys.py
validation/standard_hypo_test_demo.py
validation/xmlimport_input2/makedata.py

[henryiii]

FYI, this is PEP 597 and:

Using open(filename) to read text files encoded in UTF-8 is a common mistake. It may not work on Windows because UTF-8 is not the default encoding.

You can use -X warn_default_encoding or PYTHONWARNDEFAULTENCODING=1 to find this type of mistake.