td.py: Further re-factoring to pass flake8 checks

The output of this was confirmed the 'same' as from before this work:

 1. Generate the output.
 2. `cut -c 1-68` to a copy.
 3. diff those files.

This is necessary so as to not have 'every' line be different due to the
timestamp on it.

td.py

@@ -5,7 +5,6 @@ import time
 from collections import defaultdict
 from operator import itemgetter
 from platform import system
-from sys import platform
 
 from companion import CAPIData
 from config import applongname, appversion, config
@@ -20,6 +19,7 @@ stockbracketmap = {0: '-',
                    2: 'M',
                    3: 'H', }
 
+
 def export(data: CAPIData) -> None:
     """Export market data in TD format."""
     data_path = pathlib.Path(config.get_str('outdir'))
@@ -31,31 +31,35 @@ def export(data: CAPIData) -> None:
     with open(data_path / data_filename, 'wb') as h:
         # Format described here: https://bitbucket.org/kfsone/tradedangerous/wiki/Price%20Data
         h.write('#! trade.py import -\n'.encode('utf-8'))
-        this_platform = 'darwin' and "Mac OS" or system_name()
+        this_platform = 'darwin' and "Mac OS" or system()
         cmdr_name = data['commander']['name'].strip()
-        h.write(f'# Created by {applongname} {appversion()} on {this_platform} for Cmdr {cmdr_name}.\n'.encode('utf-8'))
-        h.write('#\n#    <item name>             <sellCR> <buyCR>   <demand>   <stock>  <timestamp>\n\n'.encode('utf-8'))
+        h.write(
+            f'# Created by {applongname} {appversion()} on {this_platform} for Cmdr {cmdr_name}.\n'.encode('utf-8')
+        )
+        h.write(
+            '#\n#    <item name>             <sellCR> <buyCR>   <demand>   <stock>  <timestamp>\n\n'.encode('utf-8')
+        )
         system_name = data['lastSystem']['name'].strip()
         starport_name = data['lastStarport']['name'].strip()
         h.write(f'@ {system_name}/{starport_name}\n'.encode('utf-8'))
 
         # sort commodities by category
-        bycategory = defaultdict(list)
+        by_category = defaultdict(list)
         for commodity in data['lastStarport']['commodities']:
-            bycategory[commodity['categoryname']].append(commodity)
+            by_category[commodity['categoryname']].append(commodity)
 
         timestamp = time.strftime('%Y-%m-%d %H:%M:%S', time.strptime(data['timestamp'], '%Y-%m-%dT%H:%M:%SZ'))
-        for category in sorted(bycategory):
-            h.write('   + {}\n'.format(category).encode('utf-8'))
+        for category in sorted(by_category):
+            h.write(f'   + {format(category)}\n'.encode('utf-8'))
             # corrections to commodity names can change the sort order
-            for commodity in sorted(bycategory[category], key=itemgetter('name')):
-                h.write('      {:<23} {:7d} {:7d} {:9}{:1} {:8}{:1}  {}\n'.format(
-                    commodity['name'],
-                    int(commodity['sellPrice']),
-                    int(commodity['buyPrice']),
-                    int(commodity['demand']) if commodity['demandBracket'] else '',
-                    demandbracketmap[commodity['demandBracket']],
-                    int(commodity['stock']) if commodity['stockBracket'] else '',
-                    stockbracketmap[commodity['stockBracket']],
-                    timestamp).encode('utf-8'))
-
+            for commodity in sorted(by_category[category], key=itemgetter('name')):
+                h.write(
+                    f"      {commodity['name']:<23}"
+                    f" {int(commodity['sellPrice']):7d}"
+                    f" {int(commodity['buyPrice']):7d}"
+                    f" {int(commodity['demand']) if commodity['demandBracket'] else '':9}"
+                    f"{demandbracketmap[commodity['demandBracket']]:1}"
+                    f" {int(commodity['stock']) if commodity['stockBracket'] else '':8}"
+                    f"{stockbracketmap[commodity['stockBracket']]:1}"
+                    f"  {timestamp}\n".encode('utf-8')
+                )