Fix epub specification violation (#113)

According to the epub
specifications (https://www.w3.org/TR/epub/#sec-zip-container-mime) epub
creators must not compress the mimetype file in the epub. The updated RemoveDrm
function creates a new zip file with using the `ZIP_DEFLATED` compression
method for all the files, except mimetype

Author: matteo-l

kobodl/koboDrmRemover.py

@@ -36,4 +36,7 @@ def RemoveDrm(self, inputPath: str, outputPath: str, contentKeys: Dict[str, str]
                     contentKeyBase64 = contentKeys.get(filename, None)
                     if contentKeyBase64 is not None:
                         contents = self.__DecryptContents(contents, contentKeyBase64)
-                    outputZip.writestr(filename, contents)
+                    if filename == "mimetype":
+                        outputZip.writestr(filename, contents, compress_type=zipfile.ZIP_STORED)
+                    else:
+                        outputZip.writestr(filename, contents)