Adjust for pandas 2.0.0

[khaeru]

Parallel to iiasa/ixmp#470; see description there for details.

[khaeru]

1. Check for any FutureWarnings from pandas 1.5.3 / pre-2.0.0 occurring in either message_ix itself or its upstream dependencies.

Here is the latest nightly run of the "pytest" CI workflow:

=============================== warnings summary ===============================
../../../../../../opt/hostedtoolcache/Python/3.10.10/x64/lib/python3.10/site-packages/jupyter_client/connect.py:20
  /opt/hostedtoolcache/Python/3.10.10/x64/lib/python3.10/site-packages/jupyter_client/connect.py:20: DeprecationWarning: Jupyter is migrating its paths to use standard platformdirs
  given by the platformdirs library.  To remove this warning and
  see the appropriate new directories, set the environment variable
  `JUPYTER_PLATFORM_DIRS=1` and then run `jupyter --paths`.
  The use of platformdirs will be the default in `jupyter_core` v6
    from jupyter_core.paths import jupyter_data_dir, jupyter_runtime_dir, secure_write

message_ix/tests/test_macro.py::test_calc_valid_years
  /home/runner/work/message_ix/message_ix/message_ix/message_ix/tests/test_macro.py:84: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
    data["gdp_calibrate"] = data["gdp_calibrate"].append(gdp_extra_yr)

message_ix/tests/tools/test_add_year.py::test_add_year
message_ix/tests/tools/test_add_year.py::test_add_year_cli
  /home/runner/work/message_ix/message_ix/message_ix/message_ix/tools/add_year/__init__.py:323: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
    yr_cat.append(

message_ix/tests/tools/test_add_year.py::test_add_year
message_ix/tests/tools/test_add_year.py::test_add_year
message_ix/tests/tools/test_add_year.py::test_add_year
message_ix/tests/tools/test_add_year.py::test_add_year
message_ix/tests/tools/test_add_year.py::test_add_year_cli
message_ix/tests/tools/test_add_year.py::test_add_year_cli
message_ix/tests/tools/test_add_year.py::test_add_year_cli
message_ix/tests/tools/test_add_year.py::test_add_year_cli
  /home/runner/work/message_ix/message_ix/message_ix/message_ix/tools/add_year/__init__.py:934: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
    df2 = df2.append(df_yr)

These appear to be multiple occurrences of the same issue: use of pandas.DataFrame.append().

[khaeru]

Note that #494 could be a vehicle for making the required changes in .tools.add_year, or they could be cherry-picked from there, etc.