Fix some but not all undefined names in scripts/__init__.py

Related to changes made in opencv#563

$ `flake8 . --count --builtins=ml_ops --select=E9,F63,F7,F82,Y --show-source --statistics`
```
./opencv-python/scripts/__init__.py:2:5: F821 undefined name 'LOADER_DIR'
    LOADER_DIR
    ^
./opencv-python/scripts/__init__.py:3:5: F821 undefined name 'PYTHON_EXTENSIONS_PATHS'
] + PYTHON_EXTENSIONS_PATHS
    ^
./opencv-python/scripts/__init__.py:15:4: F821 undefined name 'sys'
if sys.platform.startswith("linux") and ci_and_not_headless:
   ^
./opencv-python/scripts/__init__.py:16:5: F821 undefined name 'os'
    os.environ["QT_QPA_PLATFORM_PLUGIN_PATH"] = os.path.join(
    ^
./opencv-python/scripts/__init__.py:16:49: F821 undefined name 'os'
    os.environ["QT_QPA_PLATFORM_PLUGIN_PATH"] = os.path.join(
                                                ^
./opencv-python/scripts/__init__.py:17:9: F821 undefined name 'os'
        os.path.dirname(os.path.abspath(__file__)), "qt", "plugins"
        ^
./opencv-python/scripts/__init__.py:17:25: F821 undefined name 'os'
        os.path.dirname(os.path.abspath(__file__)), "qt", "plugins"
                        ^
./opencv-python/scripts/__init__.py:21:4: F821 undefined name 'sys'
if sys.platform.startswith("linux") and ci_and_not_headless:
   ^
./opencv-python/scripts/__init__.py:22:5: F821 undefined name 'os'
    os.environ["QT_QPA_FONTDIR"] = os.path.join(
    ^
./opencv-python/scripts/__init__.py:22:36: F821 undefined name 'os'
    os.environ["QT_QPA_FONTDIR"] = os.path.join(
                                   ^
./opencv-python/scripts/__init__.py:23:9: F821 undefined name 'os'
        os.path.dirname(os.path.abspath(__file__)), "qt", "fonts"
        ^
./opencv-python/scripts/__init__.py:23:25: F821 undefined name 'os'
        os.path.dirname(os.path.abspath(__file__)), "qt", "fonts"
                        ^
12    F821 undefined name 'LOADER_DIR'
12
```

scripts/__init__.py

@@ -1,3 +1,6 @@
+import os
+import sys
+
 PYTHON_EXTENSIONS_PATHS = [
     LOADER_DIR
 ] + PYTHON_EXTENSIONS_PATHS
@@ -8,7 +11,7 @@
     from .version import ci_build, headless
 
     ci_and_not_headless = ci_build and not headless
-except:
+except ImportError:
     pass
 
 # the Qt plugin is included currently only in the pre-built wheels