Use a wrapper script to bypass check-manifest if not under git

[nicoddemus]

Wrapper script is necessary because we are not under version when testing the package directly using devpi test.

Related to comment in #2051

[coveralls]

Coverage remained the same at 92.79% when pulling c0b1ceb on nicoddemus:check-manifest-script into 5717c71 on pytest-dev:master.

[nicoddemus]

Hehehe the failure:

linting runtests: commands[0] | /home/travis/build/pytest-dev/pytest/.tox/linting/bin/python scripts/check-manifest.py
lists of files in version control and sdist do not match!
missing from sdist:
  scripts
  scripts/check-manifest.py
suggested MANIFEST.in rules:
  recursive-include scripts *.py

Oh the irony! 😆

[coveralls]

Coverage remained the same at 92.79% when pulling d4b8f2c on nicoddemus:check-manifest-script into 5717c71 on pytest-dev:master.

[coveralls]

Coverage remained the same at 92.79% when pulling 351395b on nicoddemus:check-manifest-script into 71b6833 on pytest-dev:master.

[nicoddemus]

Merging as this is a minor update.