README: Fix display of "class CompressedTextField" on PyPI

[davidfstr]

On GitHub the following section displays OK as:

But on PyPI the same section displays in all capital letters when it shouldn't:

Current source code for that section reads as:

<h5><code><i>class</i> CompressedTextField(<i>encode_errors='strict', decode_errors='strict', **options</i>)</code></h5>

Suggested fix:

• Alter text to use manual <pre><code> block instead of a manual <h5>