git_sha1_gen.py: fix code style

Bare `except` are bad form as per PEP8.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5112>

bin/git_sha1_gen.py

@@ -20,7 +20,7 @@ def get_git_sha1():
             'rev-parse',
             'HEAD',
         ], stderr=open(os.devnull, 'w')).decode("ascii")
-    except:
+    except Exception:
         # don't print anything if it fails
         git_sha1 = ''
     return git_sha1