underscores in <kbd> and <samp> are incorrectly escaped, whereas underscores in <code> are correctly not escaped

[chrispy-snps]

<code>, <kbd>, and <samp> elements are all converted to Markdown code spans with single backticks.

However, underscores in <kbd> and <samp> are incorrectly escaped (they become literal backslashes in the Markdown code span), whereas underscores in <code> are correctly not escaped:

from markdownify import markdownify as md
md('<code>A_B_C</code>')    # > '`A_B_C`'
md('<kbd>A_B_C</kbd>')      # > '`A\\_B\\_C`'
md('<samp>A_B_C</samp>')    # > '`A\\_B\\_C`'