Implement worksheet methods to delete non-contiguous blocks of rows/columns in one API call (implements issue 1589)

[Ev2geny]

This implements #1589

What this PR does

Adds new Worksheet methods that delete multiple non-contiguous blocks of rows or
columns with a single batchUpdate API call, while keeping the locally cached
row_count / col_count in sync (the stale-cache problem described in the issue):

# Delete rows 3–5 and rows 8–9 in one API call
worksheet.delete_rows_blocks([(3, 5), (8, 9)])

Note: slightly expanded scope vs. what was agreed in the issue

The issue discussion agreed on delete_rows_blocks only. While implementing it I
followed the existing structure of the module, where delete_rows and
delete_columns are thin wrappers around a generic delete_dimension. Mirroring
that pattern gave the column variant essentially for free, so this PR adds:

• delete_dimension_blocks(dimension, blocks) — generic implementation
• delete_rows_blocks(blocks) — what was agreed in the issue
• delete_columns_blocks(blocks) — the symmetric column variant

Design

• Blocks are 1-based inclusive (start, end) pairs, consistent with
  delete_rows(start, end). They may be tuples or lists
  (Sequence[Sequence[int]]), in any order; they are sorted internally and the
  deleteDimension requests are emitted bottom-up, so earlier deletions in the
  batch don't shift the indexes of later ones.
• Strict validation raises ValueError before any API call: empty blocks
  list, a block without exactly 2 elements, start < 1, end < start, a block
  exceeding the current sheet size, and overlapping blocks (adjacent blocks are
  allowed).
• After the call, the cached rowCount/columnCount is decremented by the total
  number of deleted rows/columns, so subsequent add_rows()/resize() calls work
  with correct values.

Note on .. versionadded:: 6.3.0

The docstrings mark the new methods as versionadded:: 6.3.0. That version is a
guess (current release is 6.2.1 and this is a new feature, so the next minor
seemed likely). If the actual release number ends up different, these three
directives will need updating.

Testing

• Three new tests: test_delete_rows_blocks, test_delete_columns_blocks
  (both pass blocks unsorted and mixing lists/tuples) and
  test_delete_dimension_blocks_validation (all ValueError cases).
• Cassettes recorded; the full suite passes offline (tox -e py).
• The new tests were additionally verified against the live API with
  --disable-vcr.
• new API is correctly documented in HTML files after running tox -e doc

[antoineeripret]

@Ev2geny: merged :)