DEV: Ignore .venv file.
Merge pull request #1996 from jseabold/ignore-venv

DEV: Ignore .venv file.
ENH: Add api for covstruct. Clear __init__. Closes #1917.
ENH: Add api to genmod. Fix cov_struct import.
REF: Explicit imports
REF: Move cov_struct to simpler import.