BUG: Fix pd.read_html handling of rowspan in table header

[snitish]

• closes BUG: rowspan in read_html failed #60210(Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.

From the original thread:

s = '<table><tr><th rowspan="2">A</th><th>B</th></tr><tr><td>1</td></tr><tr><td>C</td><td>2</td></tr></table>'
buf = io.StringIO(s)
print(pd.read_html(buf)[0])
#    A                  B
#    A Unnamed: 1_level_1
# 0  1                NaN

# Expected:
#    A  B
# 0  A  1
# 1  C  2

The bug is due to rowspan > 1 in the header row which leads to overflow into the body rows. Current logic does not handle this case. I fix it by overflowing the partial rows from the header into the body (and similarly from body to footer if any).

[mroeschke]

Looks good. Could you add a whatsnew note in v3.0.0.rst under the I/O section?

[mroeschke]

Thanks @snitish