(maint) code clean up (#187)

## bdfr

- Add the bound instance as method parameter
- Change methods not using its bound instance to staticmethods
- Fix dangerous default argument
- Refactor the comparison involving `not`
- Refactor unnecessary `else` / `elif` when `if` block has a `raise` statement
- Refactor unnecessary `else` / `elif` when `if` block has a `return` statement
- Refactor useless `else` block in the loop
- Remove implicit `object` from the base class
- Remove reimported module
- Remove unnecessary generator
- Remove unnecessary return statement
- Remove unnecessary use of comprehension
- Remove unused imports
- Use `is` to compare type of objects
- Using not x can cause unwanted results

## Dockerfile

- use a pinned Python version tag instead of latest
- leverage cached requirements

Signed-off-by: Vladislav Doster <mvdoster@gmail.com>

Co-authored-by: Ali Parlakçı <parlakciali@gmail.com>

src/downloaders/Direct.py

@@ -1,18 +1,16 @@
 import os
 
 from src.downloaders.downloaderUtils import getFile, getExtension
-
-from src.errors import FileNameTooLong
 from src.utils import GLOBAL
-from src.utils import printToFile as print
+
 
 class Direct:
-    def __init__(self,directory,POST):
+    def __init__(self, directory, POST):
         POST['EXTENSION'] = getExtension(POST['CONTENTURL'])
-        if not os.path.exists(directory): os.makedirs(directory)
+        if not os.path.exists(directory):
+            os.makedirs(directory)
 
-        filename = GLOBAL.config['filename'].format(**POST)+POST["EXTENSION"]
-        shortFilename = POST['POSTID']+POST['EXTENSION']
+        filename = GLOBAL.config['filename'].format(**POST) + POST["EXTENSION"]
+        shortFilename = POST['POSTID'] + POST['EXTENSION']
 
-        getFile(filename,shortFilename,directory,POST['CONTENTURL'])
-        
+        getFile(filename, shortFilename, directory, POST['CONTENTURL'])