Behavior with strip_pre=mdfy.STRIP_ONE seems incorrect with trailing newlines within the pre

[stevecj]

In [1]: import markdownify as mdfy

In [2]: converter = mdfy.MarkdownConverter(strip_pre=mdfy.STRIP_ONE)

In [3]: print(converter.convert('<pre>\n\nabc\n\n</pre>'))
```

abc
```

Only one trailing newline should be stripped, not both. right?

In [4]: print(converter.convert('<pre>\n\nabc\n\n\n</pre>'))
```

abc

```

In the above, 2 of the 3 newlines are trimmed instead of just 1.