Change `isenvvar()` to only check for `=` in var names

Daemontools' envdir only checks for `=`, thus this python envdir version should have the same behavior.



Using both daemontools and python envdirs:

    $ cat foo
    bar
    $ cat _foo
    bar
    $ cat foo=bar
    foobar


    $ cat a.py
    import os
    print('foo="{}"'.format(os.getenv("foo")))
    print('_foo="{}"'.format(os.getenv("_foo")))
    print('foo=bar="{}"'.format(os.getenv("foo=bar")))


    # python envdir
    $ python -m envdir . python a.py
    foo="None"
    _foo="None"
    foo=bar="None"


    # daemontools envdir
    $ envdir . python a.py
    foo="bar"
    _foo="bar"
    foo=bar="None"