Fix for spurious whitespace in striptags output

[dairiki]

This PR restores order of operations in Markup.striptags.

Prior to 2.1.4, the order of operations in striptags was:

1. strip comments
2. strip tags
3. coalesce whitespace

In 2.1.4 the order changed to:

1. coalesce whitespace
2. strip comments
3. strip tags

As a result extra spaces were introduced in the output in cases where whitespace was separated from more whitespace by a tag or comment.

• fixes Markup.striptags: comments now get replaced with a space #417

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [n/a] Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [n/a] Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[dairiki]

Thank you, @davidism!