Environment variable names shouldn't contain the '=' character, because it often is used as a separator between a name and a value. There is an exception -- starting '=' on Windows is used for defining "hidden" environment variables.

Using names containing '=' in environment dict in subprocess is prohibited in issue30730. Proposed PR prohibits names containing '=' in two other cases: in os.putenv() and os.spawn*(). It fixes the part of issue4926.