Coincidentally I just encountered this myself when trying to activate a venv created by Python for Windows while in a Cygwin shell.  Indeed, bash for Cygwin is just like bash anywhere else, and considers it a syntax error to encounter unexpected CR's.

Alas, venv creates <path>/<to/<env>/Scripts/activate with CRLFs.  After running dos2unix on it, this script can be sourced from Cygwin and otherwise works fine.

The issue that arises with interactive mode is a known issue not specific to Python or venv.  It can also be worked around by running python with winpty.