docs: source download badge from pepy.tech to stop pypistats rate-limit errors (#88)

The download badge fetched from img.shields.io/pypi/dm/<pkg>, which shields
sources from pypistats.org. pypistats rate-limits by IP, so the badge
intermittently rendered "rate limited by upstream service".

Switch to static.pepy.tech/badge/<pkg>/month (statically cached SVG from a
different upstream) and repoint the link to pepy.tech/projects/<pkg>. Metric
stays monthly downloads.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lesnik512

README.md

@@ -8,7 +8,7 @@
 
 [![PyPI version](https://img.shields.io/pypi/v/httpware.svg)](https://pypi.org/project/httpware/)
 [![Supported Python versions](https://img.shields.io/pypi/pyversions/httpware.svg)](https://pypi.org/project/httpware/)
-[![Downloads](https://img.shields.io/pypi/dm/httpware.svg)](https://pypistats.org/packages/httpware)
+[![Downloads](https://static.pepy.tech/badge/httpware/month)](https://pepy.tech/projects/httpware)
 [![Coverage](https://img.shields.io/badge/coverage-100%25-brightgreen.svg)](https://github.com/modern-python/httpware/actions/workflows/ci.yml)
 [![CI](https://github.com/modern-python/httpware/actions/workflows/ci.yml/badge.svg)](https://github.com/modern-python/httpware/actions/workflows/ci.yml)
 [![License](https://img.shields.io/github/license/modern-python/httpware.svg)](https://github.com/modern-python/httpware/blob/main/LICENSE)