Python 3.7 began warning about open's 'U' mode. Universal newline mode (newline=None) is default in Python 3 so avoid specifying it on Python 3.

(cherry picked from commit 978a6af9)

If a.cfg extends b.cfg and both contain [a],
it was already not possible for a.cfg:[a] to combine <= & +=.
But it worked when a.cfg:[a] does not extend anything, and
commit 161fb191 broke that.

The only proper way to solve all this seems to process <= before
+= & -=. But since processing <= while extending files would cause
other issues, we'd rather process +=&-= when initializing parts.

extends can be interpreted as inheritance in OOP, but the original
behaviour was against what is commonly (always?) seen everywhere.

It is however good practice a file extends all files it needs directly
(and only them). Then if two files A & B (possibly unrelated) extends
the same third C, A was unable to overrides C values. It was even
error-prone because someone who don't use B yet could override C values
in A and later extending B would break A.

For some of our common use cases, this new algorithm is also 9x faster
(time to annotate: ~2.3s with -> ~.29s).

Other changes:
- ~/ is now expanded for non-url extends.
- An absolute (non-url) path is not longer treated like a local path
  if the base is a url.
- Better path/url normalization.

Rebase instructions:
- squash with "Chomp ../ from beginging of filenames"
- split and apply "Support ${:_profile_base_location_}." after

Revert "Cache downloaded data in zc/buildout/buildout.py:_open() in memory to accelerate remote extends."

This reverts commit 260bc0c4 partially.

The remaining should be squashed with "Write .installed.cfg only once, in safe way and only if there's any change."

We want two identical sections at different URL to be able to produce
same signature. This is feature is useful for slapos.recipe.cmmi's
shared parts.

Rebase instructions:
- squash with "apply "Support ${:_profile_base_location_}."

If we use options.items() we visit all keys and not used options cannot
be detected.

Rebase instructions:
- squash with "Add referred parts' hash strings in
  __buildout_signature__, that invokes rebuild of a part
  when one of its (recursive) dependencies are modified"
- add a comment explaining why such a lower level API is used

This is useful when using OS Python & eggs.

Useless for SlapOS.

fixup! Add referred parts' hash strings in __buildout_signature__, that invokes rebuild of a part when one of its (recursive) dependencies are modified.

The purpose is to make the part signature available sooner,
during recipe's __init__.

Commit 8be81fe4 already
broke detection of unused options in sections.

The version of Python should not affect the behaviour of a recipe.
Anyway, it was already ignored for DEVELOP_DIST eggs.

This makes the slapos mechanism to share parts more efficient. And with
the upcoming changes in buildout & slapos.recipe.cmmi, there would be
no way for the slapos.reboostrap extension to prevent everything from
being rebuilt when reboostrapping to a different version of Python,
if bootstrap parts are shared.

The monkey-patch by slapos.reboostrap now becomes useless.

When buildout encounter an error, program exists with error code 1, this
should also be the case when exiting from a debugger session when
running with -D option.

/reviewed-on !17

When a recipe is developed in a git working copy, .git administrative
database folder should be ignored, like we did with CVS and svn.

/reviewed-on !14

_install_and_load is slow, using cache saves time when there are many sections.

Contrary to bash, dash filters out environment variables
containing '-' characters.

Now specified patches are automatically applied on required eggs as well.

Useful when recipes generate non-string values to be reused by other
recipes.

  While invoke setup.py certain eggs (like scikit-learn) launch
  cetain custom builds (for cython) using subprocess and sys.executable.

  This commit aims to preserve the sys.path over the runs, even if an
  egg is using subprocess with the same python to build a component of
  the egg.

   This commit is part of fixup the "Add setup-eggs option in zc.recipe.egg:develop."