**Summary.** The `python-docx` table API is designed for _uniform_
tables (no merged cells, no nested tables). Naive processing of DOCX
tables using this API produces duplicate text when the table has merged
cells. Add a more sophisticated parsing method that reads only "root"
cells (those with an actual `<tc>` element) and skip cells spanned by a
merge.
In the process, abandon use of the `tabulate` package for this job
(which is also designed for uniform tables) and remove the whitespace
padding it adds for visual alignment of columns. Separate the text for
each cell with a single newline ("\n").
Since it's little extra trouble, add support for nested tables such that
their text also contributes to the `Table.text` string.
The new `._iter_table_texts()` method will also be used for parsing
tables in headers and footers (where they are frequently used for layout
purposes) in a closely following PR.
Fixes#2106.
In DOCX, like HTML, a table cell can itself contain a table. This is not
uncommon and is typically used for formatting purposes.
When a DOCX table is nested, create nested HTML tables to reflect that
structure and create a plain-text table with captures all the text in
nested tables, formatting it as a reasonable facsimile of a table.
This implements the solution described and spiked in PR #1952.
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Co-authored-by: Bruno Bornsztein <bruno.bornsztein@gmail.com>
