Bugfix/downgrade pip

[jaraco]

As discussed in #1644, bootstrapping issues have emerged with the pep517 build support in pip 19. This change wraps the 'install' command from tox to ensure that a requisite version of pip is present for builds, unblocking testing/development until a more permanent solution can be devised.

[jaraco]

As you can see, this patch fails on Windows, due probably to the bin/Scripts disparity or maybe due to the .exe extension.

[jaraco]

Oh, no. The cause is that pip can't uninstall itself on Windows due to locking of the executable file.

[benoit-pierre]

I was going to suggest maybe adding setuptools to the build requirements instead, so setuptools can be installed from source with pip>=19.0, but I see that we forget to include pyproject.toml in our source dist...

[jaraco]

I was going to suggest maybe adding setuptools to the build requirements instead

Actually, that's a very attractive prospect. It's always been a bit messy trying to ascertain 'which' setuptools is present during a build/install of itself. It'd sure be nice to rely on isolation and explicit declaration so that a particular build of setuptools could rely on perhaps a different version (or even build system).

[benoit-pierre]

But the command that install setuptools is separate from one to install the dependencies, so this should just work:

 tests/requirements.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git i/tests/requirements.txt w/tests/requirements.txt
index d301168a..f6719728 100644
--- i/tests/requirements.txt
+++ w/tests/requirements.txt
@@ -10,3 +10,4 @@ coverage>=4.5.1
 pytest-cov>=2.5.1
 paver; python_version>="3.6"
 futures; python_version=="2.7"
+pip==18.1

[jaraco]

Superseded by #1647