pip installation fails for older Python versions (3.8, 3.7)

[CalvinAllen]

Description

When installing older Python versions like 3.8.9, the pip installation step fails with a warning. The latest get-pip.py from https://bootstrap.pypa.io/get-pip.py no longer supports end-of-life Python versions.

Steps to Reproduce

1. Have a newer Python version installed and set as global (e.g., 3.9.9)
2. Install an older Python version: dtvem install python 3.8.9
3. Pip installation fails with warning

Error Output

✓ Python v3.8.9 installed successfully
→ Location: C:\Users\calvin.allen\.dtvem\versions\python\3.8.9
Downloading 100% |████████████████████████████████████████████████████████████| (2.2/2.2 MB, 12 MB/s)

⚠ Failed to install pip
→ You can install it manually by running: python -m ensurepip

Root Cause

The bootstrap.pypa.io/get-pip.py script dropped support for older Python versions. For EOL Python versions, we need to use version-specific URLs:

• Python 3.8: https://bootstrap.pypa.io/pip/3.8/get-pip.py
• Python 3.7: https://bootstrap.pypa.io/pip/3.7/get-pip.py
• Python 3.6: https://bootstrap.pypa.io/pip/3.6/get-pip.py

Proposed Fix

Update installPip() in the Python provider to detect the Python minor version and use the appropriate get-pip.py URL:

func (p *Provider) getPipURL(version string) string {
    parts := strings.Split(version, ".")
    if len(parts) >= 2 {
        minor, _ := strconv.Atoi(parts[1])
        if parts[0] == "3" && minor <= 8 {
            return fmt.Sprintf("https://bootstrap.pypa.io/pip/%s.%s/get-pip.py", parts[0], parts[1])
        }
    }
    return "https://bootstrap.pypa.io/get-pip.py"
}

Additional Issue: Incorrect Fallback Instructions

The current fallback message says:

→ You can install it manually by running: python -m ensurepip

This is incorrect because Windows embeddable Python packages do not include ensurepip - it's stripped out to keep the package small. Running this command results in:

C:\Users\calvin.allen\.dtvem\versions\python\3.8.9\python.exe: No module named ensurepip

The fallback instructions should be updated to:

→ To install pip manually:
→   1. Download: https://bootstrap.pypa.io/get-pip.py
→   2. Run: python get-pip.py

Acceptance Criteria

• Detect Python version and use appropriate get-pip.py URL
• Fix the fallback instructions to not reference ensurepip
• Test with Python 3.8.x, 3.9.x, and 3.12.x
• Add tests for the URL selection logic