Document use of environment markers in extras_require

PEP 496 is a draft that was never accepted, so it does not have the
correct information.

Add a no-op property for 'tag_svn_revision' to suppress errors when distutils attempts to detect and set these values based on settings in setup.cfg as found in sdists built by earlier versions of setuptools. Ref #619.

In the bootstrap script, generate a requirements.txt file and use it to install the dependencies just in time to make them available long enough to generate the egg info.

Add two more tests for _iter_code per #866, capturing the apparent expectation in the byte-code processing that's now failing on Python 3.6.

Attempt to fix issue #866 by iterating over code with `dis.Bytecode` instead of the internal `_iter_code`.

The `dis` module was already used in `_iter_code` so I figured it was safe to use `Bytecode` from it. Not sure how this assumption holds up across all supported Python releases. I can only assume `Bytecode` wasn't there before when `_iter_code` was originally written?

Note that `_iter_code` doesn't appear to be called anywhere in light of this change so I removed it.

I should also note that `get_module_constant` has never worked with `setuptools.__version__` (returns -1) because it's not a string literal; it gets that attribute from another module.  But this change does work in cases where a string literal is requested.

https://github.com/pypa/setuptools/issues/866

Intended for release to be 32.1.4, but I typed 'bumpversion minor --no-tags' instead of 'bumpversion patch --no-tags' by mistake, then tagged with what I thought I'd just bumped to.

Re-use test.paths_on_pythonpath to extend the PYTHONPATH variable rather than erasing it. When tests are run under pytest-runner (or other setup.py test invocations), the PYTHONPATH is carefully curated to include dependencies and the project under test. Overwriting PYTHONPATH will break tests in those environments. Fixes #884.

Cast the value to rmtree to bytes on Linux and Python 2 when the filesystemencoding is ascii, and let posixpath work its voodoo. Fixes #706.