Support Matplotlib 3.7 package style registration (fix #20)

[cafeco09]

• Adds root-level Matplotlib style sheets under qbstyles/ (qb-common.mplstyle, qb-dark.mplstyle, qb-light.mplstyle) so Matplotlib ≥ 3.7 can load styles via plt.style.use("qbstyles.<style>").
• Keeps existing style sheets under qbstyles/styles/ to preserve backwards compatibility with any current usage.
• Updates packaging (MANIFEST.in) so the new .mplstyle files are included in built distributions.
• Improves packaging robustness by ensuring requirements handling doesn’t hard-fail during sdist/wheel builds.

Usage (Matplotlib ≥ 3.7):

import matplotlib.pyplot as plt
plt.style.use(["qbstyles.qb-common", "qbstyles.qb-dark"])   # dark
# or
plt.style.use(["qbstyles.qb-common", "qbstyles.qb-light"])  # light

Testing / Verification:

• Built wheel locally and verified it contains:

  • qbstyles/qb-common.mplstyle, qbstyles/qb-dark.mplstyle, qbstyles/qb-light.mplstyle
  • and the original qbstyles/styles/qb-*.mplstyle files.

Fixes #20.

[cafeco09]

Built wheel locally and verified the styles are included in the distribution:
qbstyles/qb-{common,dark,light}.mplstyle and legacy qbstyles/styles/qb-*.mplstyle.
Quick test:
python -c "import matplotlib.pyplot as plt; plt.style.use(['qbstyles.qb-common','qbstyles.qb-dark']); print('OK')"