Solution.__add__: fix static reference to class; linting

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- `Solution.__add_`: Bugfix in the addition operation `+` that could cause problems with
+  child classes (i.e., classes that inherit from `Solution`) to work improperly
+
 ### Changed
 
 - Removed deprecated `pkg_resources` import in favor of `importlib.resources`

src/pyEQL/solution.py

@@ -10,8 +10,8 @@
 import math
 import os
 import warnings
-from importlib.resources import files
 from functools import lru_cache
+from importlib.resources import files
 from pathlib import Path
 from typing import Any, Literal
 
@@ -153,7 +153,7 @@ def __init__(
         if database is None:
             # load the default database, which is a JSONStore
             db_store = IonDB
-        elif isinstance(database, (str, Path)):
+        elif isinstance(database, str | Path):
             db_store = JSONStore(str(database), key="formula")
             logger.info(f"Created maggma JSONStore from .json file {database}")
         else:
@@ -2513,7 +2513,7 @@ def __add__(self, other: Solution):
         mix_pE = -math.log10((mol_e_self + mol_e_other) / mix_vol.to("L").magnitude)
 
         # create a new solution
-        return Solution(
+        return self.__class__(
             mix_species.data,  # pass a regular dict instead of the FormulaDict
             volume=str(mix_vol),
             pressure=str(mix_pressure),