Fix BlockIO issues

[mxmlnkn]

Closes #3

I got a bit distracted while trying to add the tests. Feel free to cherry-pick what you need.

[mxmlnkn]

These are the pylint errors that look concerning but as I don't know how to fix them, I leave this to you:

************* Module ext4.enum
ext4/enum.py:30:4: E1139: Invalid metaclass 'EnumerationType' used (invalid-metaclass)
************* Module ext4.inode
ext4/inode.py:152:19: E1120: No value for argument 'cls' in classmethod call (no-value-for-parameter)
************* Module ext4.xattr
ext4/xattr.py:47:20: E1120: No value for argument 'offset' in constructor call (no-value-for-parameter)
ext4/xattr.py:47:20: E1120: No value for argument 'size' in constructor call (no-value-for-parameter)
ext4/xattr.py:71:34: E0606: Possibly using variable 'value' before assignment (possibly-used-before-assignment)

[Eeems]

This does appear to be a lot more than what I was asking for, as well as some large opinionated changes to the project that are not really proper etiquette to do without asking permission first.

I was expecting only the following changes:

• fixes to BlockIO based on the issues outlined in File objects seekable returns False even though they are seekable and seek returns None #3
• addition of some very simple tests ensuring that these changes work to the existing test.py

Because it's much larger, and you've left it to me to cherry pick, it will probably be a while before I get to it, and detangle what I want to include.

[mxmlnkn]

I have extracted the bugfix for #3. I'll open another PR for the Python and dependency requirements, which I think are important. The rest of the dropped commits to expand the CI can be found here and may be cherry-picked.