uniquify paths in PYTHONPATH

When reading config files, require them to be encoded with UTF-8.

Update changelog for version 40.9.0

Include path in error message when version is missing (Fixes #1664)

Related to pip's github issue pypa/pip#6194.

This has come up in pip's issue tracker (github) multiple times:

  - pypa/pip#6177
  - pypa/pip#6283
  - pypa/pip#6194

Update pypy in travis

There is some issue with pypy2.7-5.8.0 in Travis. This probably does not
solve the *root* issue, but updating the pypy version does seem to fix
the build, so in the interest of unblocking the project, we will merge
this change.

Support requirement parsing in setuptools.build_meta

Since pkg_resources is imported elsewhere anyway, we don't get much
value out of porting the requirement parser locally.

This fixes GH #1682 by porting the pkg_resources requirement parsing
logic into setuptools.build_meta, so that all valid requirement
specifiers passed to setup_requires will be added to the
get_requires_for_build_* function outputs.

Fixes GH #1682

Per GH #1682, with setuptools.build_meta we are not properly handling
the situation where setup_requires is actually a newline-delimited
string rather than a list, which is supported by setup.py interface.

This adds several failing (and some passing) tests for how
setup_requires is handled by setuptools.build_meta.

Found by lgtm

When running in a complex environment with lots of installed
packages, PYTHONPATH gets way too long. Instead, just make sure
that paths_on_pythonpath doesn't contain duplicates

These sections of the documentation were never written, and refer to deprecated functionality anyway, so they can be removed.

tests: improve `test_pip_upgrade_from_source`

Parametrize the test to check different versions of pip
(including master) are correctly supported.

Move version to declarative config

Improve badges in README

Move bumpversion config to .bumpversion.cfg as it seems not to be possible to handle setup.cfg. Ref c4urself/bump2version#62.