Offload more work to the underlying pip command used to install the
build requirements, so there's no need to duplicate code to handle
environment markers/extras. This is done by setting the correct options
from the finder passed in argument to `_install_build_reqs`.

- avoid hitting the index: use `common_wheels`/`script.pip_install_local`
- use `script.pip(..., use_module=True)` to simplify some tests
- improve `test_pep518_uses_build_env` parametrization
- simplify `test_pep518_with_user_pip`: we only need to check build
  requirements can be installed, so no need for testing with both
  `install` and `wheel` command
- fix `test_pip_wheel_with_pep518_build_reqs_no_isolation`: building
  pep518-3.0 without isolation should fail if the build requirements
  are not installed.

- change module name to prevent conflicts with other distributions
- add a `__version__` field for version checks

Keep only one selfcheck.json file

Drop support for Python 3.3

Start using new GitHub support for issue templates

Fix a rare segfault

If pip10 is installed and pyopenssl+cryptography are present in the
environment pip's vendored urllib3 will automatically activate its
pyopenssl adapter. This is by design so users can potentially opt-in to
more up-to-date TLS versions than what are provided by their copy of
Python. However, if a user has these packages & subsequently runs
pip install --ignore-installed cryptography then a segfault will occur
due to overwriting an mmap'd shared object.

This commit introduces 2 issue "types", feature request and bug report.

Ref: https://blog.github.com/2018-05-02-issue-template-improvements/

The flag makes pip prefer an older but valid binary distributions over a newer source distributions.

Fixes #3785.

Fix the capitalization of "GitHub".

enable pip cache in appveyor and travis builds

Add a note to the docs about cosmetic changes in PRs

Revendoring script not to rewrite binascii import

It contains the executables: easy_install, distro, chardetect etc.

It should not be picked as vendored library.

Related to https://github.com/pypa/pip/issues/5347

If there is an import from hamster, while we vendor ham, we should
not rewrite it.

Fixes https://github.com/pypa/pip/issues/5347

Add the ability to short circuit Travis CI builds for docs-only changes

Improve Development docs