ScriptFileFormula's use is that it installs whatever was downloaded
to bin; if the install is overridden, there's no benefit over deriving
directly from Formula.

Enumerable#max_by didn't exist in 1.8.6, so linkapps failed on Leopard.

Setting LDFLAGS to `-F HOMEBREW_PREFIX/opt/python/Frameworks`
breaks at least the scipy build. Perhaps others, too.

* The python do ... end block does no longer
  require 'superenv' but saves and restores
  the ENV by using ENV.to_hash and `ensure`.
  This should resolve some build problems
  with formulae using `env :std`!
* `python_helper` is now in a module `Python`.
* Imporoved some comments in python_dependency.rb

Tokens like "b4", "beta1", "p195", &c. are now treated as atoms rather
than being broken down even further. Additionally, we enable support for
padding in the middle of versions strings, so we can successfully
compare something like "2.1-p195" with "2.1.0-p194" by inferring that
"2.1" is really "2.1.0".

This fixes the comparison "9.9.3-P1" > "9.9.3" which previously has not
been handled correctly.

Closes Homebrew/homebrew#20261.

This applies only to non-brewed python:
The user will still have to set his PYTHONPATH
in order to import python modules in an
external python interpreter. However, a
warning will be displayed, now.

Further, during the build, the PYTHONPATH
will be set to Homebrew's global site-packages
so that software (like PyQt and others) that
need to `import sipconfig` in python can
find the brewed sip and its python module.

Since Python has been removed from superenv and
added as an explicit `depends_on :python`,
we should add
`-F#{HOMEBREW_PREFIX}/opt/python/Framewoks`
so that build tools that don't use
`python-config --ldflags` (as they should!)
can link against brewed Python.

The depends_on 'nose' => :python

tests fails even if people did `pip install nose`
with their brewed pythons because during
the tests the PATH is set up such that
/usr/bin comes before HOMEBRE_PREFIX/bin.

This is a hot fix. Proper fix need resorting
the PATH.

Closes Homebrew/homebrew#20241.
Closes Homebrew/homebrew#20248.

Now it is only shown for an `python do ... end` block
and not for ordinary python.site_packages or other
methods.

Closes Homebrew/homebrew#20159.

Closes Homebrew/homebrew#20239.

sys.version_info.major is only possible on 2.7+ or so.
Therefore, we now use [0] instead.

Also allow to build with --HEAD by making brew's
download strategy aware of a brewed but unlined hg.

Adapted the caveats so that the procedure to brew
hg --HEAD is clear.

Mention how to config hgk in the caveats.

References Homebrew/homebrew#20142.

An "optional build-time dep" is unlikely, but possible, and I happen to
notice this wasn't handled correctly while digging around in the deps
code over the last few days.

Closes Homebrew/homebrew#19731.

This reverts commit 9aaa89dbe832387249ab3ea4e22a8018cc9fe30a.

Fixes Homebrew/homebrew#19857.

System python (for 2.x) does not provide `python2` binary.
So use `python` again.

For Python 3.x we still use `python3`.

This breaks my test runner in weird ways.

To support system python on OS X