Make djnz instruction to return nz condition

Since djnz is dec b; jr nz, we signal this instruction uses nz.
Note this has a slightly different semantic:
For call, ret, jr, jp this is the condition under the instruction
executes. For djnz, the instruction always does dec b, so it
"partially executes".

Also do some code cleanup to pass flake8 tests.

Author: boriel

arch/zx48k/optimizer/asm.py

@@ -135,17 +135,21 @@ def condition(asm):
         or None. E.g. 'c' for Carry, 'nz' for not-zero, etc.
         That is the condition required for this instruction
         to execute. For example: ADC A, 0 does NOT have a
-        condition flag (it always execute) whilst RETC does.
+        condition flag (it always execute) whilst RET C does.
+        DJNZ has condition flag NZ
         """
         i = Asm.inst(asm)
 
-        if i not in {'call', 'jp', 'jr', 'ret'}:
+        if i not in {'call', 'jp', 'jr', 'ret', 'djnz'}:
             return None  # This instruction always execute
 
         if i == 'ret':
             asm = [x.lower() for x in asm.split(' ') if x != '']
             return asm[1] if len(asm) > 1 else None
 
+        if i == 'djnz':
+            return 'nz'
+
         asm = [x.strip() for x in asm.split(',')]
         asm = [x.lower() for x in asm[0].split(' ') if x != '']
         if len(asm) > 1 and asm[1] in {'c', 'nc', 'z', 'nz', 'po', 'pe', 'p', 'm'}:
@@ -208,7 +212,7 @@ def make_patt(mnemo):
         """
         return r'^[ \t]*{}[ \t]*$'.format(RE_.sub('.+', re.escape(mnemo).replace(',', r',[ \t]*')))
 
-    RE_=re.compile(r'\bN+\b')
+    RE_ = re.compile(r'\bN+\b')
     for mnemo, opcode_data in z80.Z80SET.items():
         pattern = make_patt(mnemo)
         Z80_PATTERN[re.compile(pattern, flags=re.IGNORECASE)] = opcode_data