Adjust format_mass() for pint 0.18

pint.formatting.format_unit() changed slightly between pint 0.17 and
0.18; see hgrecco/pint@8c719483, part of hgrecco/pint#1371.

- Discard the "~" format modifier once it has been checked.
- Discard any leading ":".
- Adjust docstring.

iam_units/__init__.py

@@ -111,8 +111,8 @@ def format_mass(obj, info, spec=None):
  info : str
  Any information, e.g. the symbol of a GHG species.
  spec : str, optional
- Pint formatting specifier such as ':H' (HTML format), ':~' (compact
- format with symbols), etc.
+ Pint formatting specifier such as "H" (HTML format), "~C" (compact format with
+ symbols), etc.
  """
  spec = spec or obj.default_format
 
@@ -122,15 +122,20 @@ def format_mass(obj, info, spec=None):
  except AttributeError:
  pass # Already a Unit object
 
- # Use the symbol for a ':~' spec
+ # Use the symbol if the modifier "~" is in `spec`; else the canonical name. cf.
+ # pint.Unit.__format__()
  method = registry._get_symbol if "~" in spec else lambda k: k
  # Collect the pieces of the unit expression
  units = [[method(key), value] for key, value in obj._units.items()]
 
  # Index of the mass component
  mass_index = list(obj.dimensionality.keys()).index("[mass]")
- # Append the information to the mass component
+ # Append the information (e.g. species) to the mass component
  units[mass_index][0] += f" {info}"
 
- # Hand off to pint's formatting
- return format_unit(to_units_container(dict(units), registry=registry), spec)
+ # Prepare pint.util.UnitsContainer and hand off to pint's formatting. Discard "~"
+ # (used above) and ":" (invalid in pint ≥0.18).
+ return format_unit(
+ to_units_container(dict(units), registry=registry),
+ spec.replace("~", "").lstrip(":"),
+ )