FIX: git and hg revision checkout under Windows

Checking for new implementation solving issue #1740

Windows does not change the working directory for a process via `cd .. && <process>` (see e.g. this question: https://stackoverflow.com/q/55641332/8575607 ). This commit replaces the use of `cd .. &&` with arguments provided by `git` and `hg` respectively.

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`