view-user --parsable: improve output formatting

Problem: Since the formatting of the columns in the output of view-user
--parsable is dependent on the length of the longest column name, the
width of every column is forced to match, which can result in some ugly
screen wrapping even in a relatively small font.

Improve the view-user --parsable formatting option by making the width
of each column dynamic based on the length of each column name instead
of based on the length of the longest column name.

src/bindings/python/fluxacct/accounting/user_subcommands.py

@@ -130,21 +130,26 @@ def create_json_object(conn, user):
 
 
 def get_user_rows(conn, user, headers, rows, parsable, json_fmt):
-    user_str = ""
-
     if parsable is True:
-        # find length of longest column name
-        col_width = len(sorted(headers, key=len)[-1])
+        # fetch column names and determine width of each column
+        col_widths = [
+            max(len(str(value)) for value in [col] + [row[i] for row in rows])
+            for i, col in enumerate(headers)
+        ]
 
-        for header in headers:
-            user_str += header.ljust(col_width)
-        user_str += "\n"
-        for row in rows:
-            for col in list(row):
-                user_str += str(col).ljust(col_width)
+        def format_row(row):
+            return " | ".join(
+                [f"{str(value).ljust(col_widths[i])}" for i, value in enumerate(row)]
+            )
 
-        return user_str
+        header = format_row(headers)
+        separator = "-+-".join(["-" * width for width in col_widths])
+        data_rows = "\n".join([format_row(row) for row in rows])
+        table = f"{header}\n{separator}\n{data_rows}"
+
+        return table
 
+    user_str = ""
     if json_fmt is True:
         user_str += create_json_object(conn, user)