fix: remove use of deprecated cgi module (#1006)

Co-authored-by: Andrew Gorcester <gorcester@google.com>

google/cloud/storage/blob.py

@@ -18,7 +18,6 @@
 """
 
 import base64
-import cgi
 import copy
 import hashlib
 from io import BytesIO
@@ -27,6 +26,7 @@
 import mimetypes
 import os
 import re
+from email.parser import HeaderParser
 from urllib.parse import parse_qsl
 from urllib.parse import quote
 from urllib.parse import urlencode
@@ -1628,7 +1628,8 @@ def download_as_text(
             return data.decode(encoding)
 
         if self.content_type is not None:
-            _, params = cgi.parse_header(self.content_type)
+            msg = HeaderParser().parsestr("Content-Type: " + self.content_type)
+            params = dict(msg.get_params()[1:])
             if "charset" in params:
                 return data.decode(params["charset"])