REF: Rename jac -> score_obs. Closes #1785.
MAINT: Deprecate jac methods.
Merge pull request #1982 from jseabold/jac-rename

REF: Rename jac -> score_obs. Closes #1785.
Fix issue #1993
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.
Merge pull request #1981 from jseabold/cov_struct-import

ENH: Add api for covstruct. Clear __init__. Closes #1917.
Merge pull request #1995 from kshedden/mixedlm_1993

BUG: MixedLM list instead of array closes #1993
BUG/ENH: WLS generic robust cov_type didn't use whitened, see #1896
enable tests
TST: commented out cases in tests