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.
ENH: Wrap X12/X13 AUTOMDL. Closes #442.
ENH: Suppport x12 and x13 binary names
TST: Add some smoke tests for x12/x13
DOC/ENH: Add docstrings. Allow DataFrames. Fix transform
REF: X12->X13. Change function names
REF: Rearrange arguments
REF: Return a Bunch object.
REF: select_arima_order -> arima_select_order
DOC: Add note about X12/X13 optional dependency
DOC: Add x13 functions
ENH: Add results object with plot method
DOC: Add X12/X13 to release notes