Fix all critical, major, minor, and nit code quality issues

[bjinwright]

Critical:

• Replace mutable default validators=[] with None across all init signatures (C1)
• Replace mutable default errors=dict() with None in render() (C2)
• Remove self.validators = set(self.validators) to preserve validator order (C3)
• Fix render_static_assets() to use dict.fromkeys() for ordered deduplication (C4)
• Rewrite setup.py to drop the removed pip.req.parse_requirements() API (C5)

Major:

• Replace wildcard 'from valley.mixins import *' with explicit valley.properties imports (M1)
• Fix CSS class joining from comma-separated to space-separated (M2)
• Fix copy-paste bug in test_booleanfield using StringField for optional case (M3)
• Add comprehensive Form tests covering iteration, render, and static assets (M4)
• Add missing envs dependency to pyproject.toml and requirements.txt (M5)

Minor:

• Quote placeholder attributes in base.html and textarea.html templates (N1, N2)
• Make TextAreaField rows/cols configurable via init params (N3)
• Update CKEditor CDN from v4.6.2 to v4.22.1 (N4)
• Align setup.py version to 1.3.1 and license to Apache-2.0 (N5, N6)
• Remove deprecated six (Python 2 compat) from requirements.txt (N7)

Nit:

• Add type hints to all method signatures in fields.py, form.py (T1)
• Restore BUILTIN_DOC_ATTRS with type annotation (valley contract) (T2)
• Rename loop variable i -> package in utils.py (T3)
• Configure bumpversion to update pyproject.toml version (T4)
• Update README Python version list from EOL (2.7/3.3/3.4/3.5) to 3.8-3.11 (T5)

All 23 tests pass.

https://claude.ai/code/session_01Jgs6rcgzZdwESjA2TzATNV