Rows containing cells skipping reference attribute are incorrectly parsed

Some Excel files generated with third-party libraries such as SpreadsheetGear - there could be other, skip the "r" attribute on "c" tags within rows. Excel seems to put this attribute on every cell, but I couldn't find any evidence that the standard commands it.

openpyxl is not expecting these attributes to be skipped, so the result is that we end up with an invalid parsed row.

Impacted rows look like this:

<row xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
    <c>
        <v>1</v>
    </c>
    <c r="D1">
        <v>2</v>
    </c>
    <c>
        <v>3</v>
    </c>
    <c r="G1">
        <v>4</v>
    </c>
</row>

Expected

• Cell B1 has no value
• Cell E1 has value 3

Actual

• Cell B1 has value 3
• Cell E1 has no value

I have a fix in hand, and will try to submit a pull-request referencing this issue.