SCons: Format buildsystem files with psf/black

Configured for a max line length of 120 characters.

psf/black is very opinionated and purposely doesn't leave much room for
configuration. The output is mostly OK so that should be fine for us,
but some things worth noting:

- Manually wrapped strings will be reflowed, so by using a line length
  of 120 for the sake of preserving readability for our long command
  calls, it also means that some manually wrapped strings are back on
  the same line and should be manually merged again.

- Code generators using string concatenation extensively look awful,
  since black puts each operand on a single line. We need to refactor
  these generators to use more pythonic string formatting, for which
  many options are available (`%`, `format` or f-strings).

- CI checks and a pre-commit hook will be added to ensure that future
  buildsystem changes are well-formatted.

(cherry picked from commit cd4e46ee65)

misc/scripts/fix_headers.py

@@ -37,24 +37,24 @@ files = open("files", "r")
 
 fname = files.readline()
 
-while (fname != ""):
+while fname != "":
 
     # Handle replacing $filename with actual filename and keep alignment
     fsingle = fname.strip()
-    if (fsingle.find("/") != -1):
-        fsingle = fsingle[fsingle.rfind("/") + 1:]
+    if fsingle.find("/") != -1:
+        fsingle = fsingle[fsingle.rfind("/") + 1 :]
     rep_fl = "$filename"
     rep_fi = fsingle
     len_fl = len(rep_fl)
     len_fi = len(rep_fi)
     # Pad with spaces to keep alignment
-    if (len_fi < len_fl):
+    if len_fi < len_fl:
         for x in range(len_fl - len_fi):
             rep_fi += " "
-    elif (len_fl < len_fi):
+    elif len_fl < len_fi:
         for x in range(len_fi - len_fl):
             rep_fl += " "
-    if (header.find(rep_fl) != -1):
+    if header.find(rep_fl) != -1:
         text = header.replace(rep_fl, rep_fi)
     else:
         text = header.replace("$filename", fsingle)
@@ -71,21 +71,21 @@ while (fname != ""):
     line = fileread.readline()
     header_done = False
 
-    while (line.strip() == ""): # Skip empty lines at the top
+    while line.strip() == "":  # Skip empty lines at the top
         line = fileread.readline()
 
-    if (line.find("/**********") == -1): # Godot header starts this way
+    if line.find("/**********") == -1:  # Godot header starts this way
         # Maybe starting with a non-Godot comment, abort header magic
         header_done = True
 
-    while (not header_done): # Handle header now
-        if (line.find("/*") != 0): # No more starting with a comment
+    while not header_done:  # Handle header now
+        if line.find("/*") != 0:  # No more starting with a comment
             header_done = True
-            if (line.strip() != ""):
+            if line.strip() != "":
                 text += line
         line = fileread.readline()
 
-    while (line != ""): # Dump everything until EOF
+    while line != "":  # Dump everything until EOF
         text += line
         line = fileread.readline()