Fix utils.encoding.auto_decode() LookupError  with invalid encodings

utils.encoding.auto_decode() was broken when decoding Big Endian BOM
byte-strings on Little Endian or vice versa.

The TestEncoding.test_auto_decode_utf_16_le test was failing on Big Endian
systems, such as Fedora's s390x builders. A similar test, but with BE BOM
test_auto_decode_utf_16_be was added in order to reproduce this on a Little
Endian system (which is much easier to come by).

A regression test was added to check that all listed encodings in
utils.encoding.BOMS are valid.

Fixes https://github.com/pypa/pip/issues/6054

Bazaar: Export directly from the remote branch.

Remove extra `.` in docs

Whether to build wheels is no longer affected by --no-cache-dir.

Reject --build-options for PEP 517 builds

Prefix warning and error log messages

Remove note about flake8 noqa comments

Fix #5499: Include in pip's User-Agent whether it looks like pip is in CI

Add format_command_args().

The builtin has been available since Python 2.6. Makes the code slightly
more forward compatible by removing an unnecessary use of six.

Use link.is_artifact property instead of scheme checking

delete some dead code

Mark 3 tests as network tests

Update flake8 to 3.7.6