feature: make Rows sliceable

[hexum]

When slice of rows is gotten, the result object is not iterable.

import docx
d = docx.Document()
d.add_table(5,5)
rs = d.tables[0].rows[1:3]
for row in rs:
    print(row)

----> 1 for row in rs:
      2     print(row)
      3 

TypeError: '_Row' object is not iterable

[scanny]

What's happening is actually that Table.rows is not sliceable yet, so what happens is you get back a single row object rather than a sequence. As a workaround you could use this for the time being:

row_slice = [row for row in table.rows][1:3]
for row in row_slice:
    print(row)

I'll retitle this issue as a feature request to make the _Rows object sliceable.

[scanny]

Added in v0.8.0.