open file using context manager to properly close it again (#381)

linopy/solvers.py

@@ -162,10 +162,12 @@ def path_to_string(path: Path) -> str:
 
 
 def read_sense_from_problem_file(problem_fn: Path | str):
-    f = open(problem_fn).read()
-    if read_io_api_from_problem_file(problem_fn) == "lp":
+    with open(problem_fn) as file:
+        f = file.read()
+    file_format = read_io_api_from_problem_file(problem_fn)
+    if file_format == "lp":
         return "min" if "min" in f.lower() else "max"
-    elif read_io_api_from_problem_file(problem_fn) == "mps":
+    elif file_format == "mps":
         return "max" if "OBJSENSE\n  MAX\n" in f else "min"
     else:
         msg = "Unsupported problem file format."