Fix 6 Flake8 issues (#6541)

* F541

* F821

* F841

* E741

* E302

* E722

* Apply suggestions from code review

* Update general.py

* Update datasets.py

* Update export.py

* Update plots.py

* Update plots.py

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/downloads.py

@@ -62,12 +62,12 @@ def attempt_download(file, repo='ultralytics/yolov5'):  # from utils.downloads i
             response = requests.get(f'https://api.github.com/repos/{repo}/releases/latest').json()  # github api
             assets = [x['name'] for x in response['assets']]  # release assets, i.e. ['yolov5s.pt', 'yolov5m.pt', ...]
             tag = response['tag_name']  # i.e. 'v1.0'
-        except:  # fallback plan
+        except Exception:  # fallback plan
             assets = ['yolov5n.pt', 'yolov5s.pt', 'yolov5m.pt', 'yolov5l.pt', 'yolov5x.pt',
                       'yolov5n6.pt', 'yolov5s6.pt', 'yolov5m6.pt', 'yolov5l6.pt', 'yolov5x6.pt']
             try:
                 tag = subprocess.check_output('git tag', shell=True, stderr=subprocess.STDOUT).decode().split()[-1]
-            except:
+            except Exception:
                 tag = 'v6.0'  # current release
 
         if name in assets: