Backend should not say that setuptools is needed to build

Fix issue with missing author metadata

Prior to this patch, if the author or author_email were omitted from
`setup`, a malformed `PKG-INFO` would be created.

Fix Travis test for pyc files in egg-info

`!` is a special character in YAML syntax, so this was being interpreted
as `grep pyc ...`, not `! grep pyc ...`

I've also added the same `pyc` test to the normal tests.

Release 40.6.0

Fix upload metadata

This comment is not used anywhere and `platform.dist()` is deprecated.

See CPython PR #10414: https://github.com/python/cpython/pull/10414
and bpo-35186: https://bugs.python.org/issue35186

Call os.path.normpath to normalize paths for comp

The `upload.show_response` feature was not added until Python 3. Rather
than backport it, it is now enabled only if supported.

This also adds a "smoke test" for the feature.

This fixes uploads when bdist_rpm or bdist_dumb are the command, both of
which insert a comment about what platform they are built for.

This fixes an issue where `distutils.spawn.spawn` was not available in
the ported upload_file, which is only used when signing the data.

This also adds a test that the gpg signature command is invoked and
included in the uploaded data.

`test_upload_metadata` was written before the fixture, so this updates
the test to use the fixture.

This is a fixture to create an upload command with a patched version of
urlopen so that no HTTP queries are sent.

Rather than writing to a file in a temporary directory, we can write to
and read from an in-memory buffer, now that the encoding functionality
in write_pkg_file is fixed.

This creates a wrapper function for writing fields in the PKG-INFO file,
both to simplify the syntax and to add a point where we can inject an
encoding function in order to support Python 2.7 compatibility.

Previously this value was hard-coded to '1.0', which was inaccurate for
many packages.

Fixes #1381

This turns get_metadata_version into a method on DistributionMetadata,
populated either by inferrence (in the case of package metadata
specified in `setup`) or from the data in a specified PKG-INFO file.

To populate metadata_version from PKG-INFO, we need to monkey patch
read_pkg_file in addition to write_pkg_file.