Support Python 3.11

[khaeru]

In the test suite for iiasa/message_ix, we see the following error with Python 3.11 in a test that uses pyam.IamDataFrame.plot():

/tmp/ipykernel_8873/3284970234.py in ?()
      1 get_ipython().run_line_magic('matplotlib', 'inline')
      2 
----> 3 (df.filter(model="Westeros Electrified", scenario="baseline", region="Westeros").plot())

/opt/hostedtoolcache/Python/3.11.3/x64/lib/python3.11/site-packages/pyam/plotting.py in ?(self, kind, *args, **kwargs)
    103     def __call__(self, kind="line", *args, **kwargs):
--> 104         return getattr(self, kind)(**kwargs)

/opt/hostedtoolcache/Python/3.11.3/x64/lib/python3.11/site-packages/pyam/plotting.py in ?(self, **kwargs)
    106     def line(self, **kwargs):
--> 107         return line(self._parent, **kwargs)

/opt/hostedtoolcache/Python/3.11.3/x64/lib/python3.11/site-packages/pyam/plotting.py in ?(df, x, y, order, legend, title, color, marker, linestyle, fill_between, final_ranges, rm_legend_label, ax, cmap, **kwargs)
   1051     # pop label to avoid multiple values for plot-kwarg
   1052     label = kwargs.pop("label", None)
   1053 
   1054     # plot data, keeping track of which legend labels to apply
-> 1055     for col, data in df.iteritems():
   1056         # handle case where columns are not strings or only have 1 dimension
   1057         col = list(map(str, to_list(col)))
   1058         pargs = {}

/opt/hostedtoolcache/Python/3.11.3/x64/lib/python3.11/site-packages/pandas/core/generic.py in ?(self, name)
   5985             and name not in self._accessors
   5986             and self._info_axis._can_hold_identifiers_and_holds_name(name)
   5987         ):
   5988             return self[name]
-> 5989         return object.__getattribute__(self, name)

AttributeError: 'DataFrame' object has no attribute 'iteritems'

I notice that python_requires = >=3.7, <3.11 was set in setup.cfg, apparently in #654: https://github.com/IAMconsortium/pyam/blame/947d8c4df2dcb362baed6e1274d63a24142ba067/setup.cfg#L22

This is low urgency: we can simply caution message_ix users with Python 3.11 to avoid using this feature of pyam or to use Python 3.10 instead. I open the issue mainly so we can track the reason for xfailing tests in message_ix.

[danielhuppmann]

This issue does not seem to be directly related to Python 3.11, or at least there is not test for the feature - see #745. Can you point me to the file where this error originates?