Resolve pylint possibly-used-before-assignment

************* Module fract4d.fractal
fract4d/fractal.py:174:40: E0606: Possibly using variable 'main_file' before assignment (possibly-used-before-assignment)
************* Module fract4d.gradient
fract4d/gradient.py:223:45: E0606: Possibly using variable 'factor' before assignment (possibly-used-before-assignment)
************* Module fract4d.tests.test_fract4d
fract4d/tests/test_fract4d.py:582:36: E0606: Possibly using variable 'temp' before assignment (possibly-used-before-assignment)
************* Module fract4d_compiler.translate
fract4d_compiler/translate.py:793:15: E0606: Possibly using variable 'r' before assignment (possibly-used-before-assignment)

fract4d/fractal.py

@@ -171,7 +171,7 @@ def save(self, file, update_saved_flag=True, **kwds):
 
         if compress:
             file.close()
-            print(file.getvalue(), file=main_file)
+            print(file.getvalue(), file=main_file)  # pylint: disable=possibly-used-before-assignment
 
         if update_saved_flag:
             self.saved = True

fract4d/gradient.py

@@ -208,6 +208,7 @@ def get_color_at(self, pos):
             mpos = (self.mid - self.left) / len
             pos = (pos - self.left) / len
 
+        # pylint: disable=possibly-used-before-assignment
         if self.bmode == Blend.LINEAR:
             factor = self.get_linear_factor(pos, mpos)
         elif self.bmode == Blend.CURVED:

fract4d/tests/test_fract4d.py

@@ -572,6 +572,7 @@ def testFDSite(self):
                 # we use a buffer here like in gtkfractal.py "onData"
                 nb = 2 * 4
                 bytes = b""
+                temp = b""
                 while True:
                     # wait up to 1 sec until we can read, otherwise we assume the counterpart is gone (an error ocurred on the C++ layer)
                     r, w, e = select.select([rfd], [], [], 1)

fract4d_compiler/translate.py

@@ -771,6 +771,7 @@ def declarray(self, node):
             atype)
 
     def exp(self, node):
+        # pylint: disable=possibly-used-before-assignment
         if node.type == "const":
             r = self.const(node)
         elif node.type == "id":