Version 1.0.3

Fixed an issue with Container and ZipFile in Python pre-3.8.

When the name of a file added to container contained a non-ASCII character,
a consequent read() of this file failed with an error:
```
zipfile.BadZipFile: File name in directory 'FILE' and header b'FILE' differ.
```
After closing the ZipFile, reading files was OK.

As a solution, now the container is opened in read-only mode and all write operations
are performed after closing the read-only zipfile, opening it in append mode and
closing it again.
Added a public property zip_file to the Container instance which uses a cached
read only zip file handle, opening it if necessary.
Added a public property zip_writer to the Container instance which opens
an appendable zip file handle, clearing the zip_file cache.
Merged the verify_container and _verify_container_contents methods
because the latter made little sense on its own as a private method.
Replaced _add_mimetype() with _create_container() which creates and initializes
the buffer in one single place.
[#7] Added compatibility with Latvian edoc format - @vgaicuks

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