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This build makes sure everything still works without optional
dependencies (numpy/scipy/eigen) and also tests the automatic
discovery functions in CMake (Python version, C++ standard).

[skip appveyor]

Fix test suite failure without numpy and improve module init diagnostics

Fix dtype::strip_padding() on Intel compiler

Fixes #357.

Add support for iterators with different begin/end types

- ICPC can't handle the NCVirt trampoline which returns a non-copyable
  type, which is likely due to a constexpr/SFINAE issue. This disables
  the test on that compiler so that at least the rest can be tested.

Fix int_ shadowing problem in detail namespace

If operators.h is included, int_ function in the `detail`
namespace will shadow pybind11::int_ type, so the fully qualified
name has to be used.

Workaround for random failure of pytest capture on Windows

pytest can capture test output both globally (controlled by the cmd line
flag --capture) or locally (`capsys` and `capfd` fixtures). Enabling both
methods at the same time causes problems on Windows: test output is not
captured sometimes, resulting in test failure. This happens seemingly at
random.

This workaround disables global output capture ("-s", i.e. "--capture=no")
leaving only the local capture fixtures. As a side-effect test output on
AppVeyor CI is a little messy, but this will have to do until a better
solution is found.

Port test suite to pytest

Most of the test code is left in C++ since this is the
intended use case for the eval functions.

Test compilation instructions for Windows were changed to use the
`cmake --build` command line invocation which should be easier than
manually setting up using the CMake GUI and Visual Studio.

Pytest is a development dependency but we can make it painless by
automating the install using CMake.

There are more enum tests than 'constants and functions'.

The C++ part of the test code is modified to achieve this. As a result,
this kind of test:

```python
with capture:
    kw_func1(5, y=10)
assert capture == "kw_func(x=5, y=10)"
```

can be replaced with a simple:

`assert kw_func1(5, y=10) == "x=5, y=10"`

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

Support keep_alive where nurse may be None

Build docs just once, in a dedicated build

For example keep_alive<0,1>() should work where the return value may sometimes be None. At present a "Could not allocate weak reference!" exception is thrown.
Update documentation to clarify behaviour of keep_alive when nurse is None or does not support weak references.

Rather than adding an `if [ -n "$DOCS" ]` as a separate install
instruction, this simplifies the travis-ci logic to do the pip/venv
setup in the `before_install` hook, leaving the install hook to just
install the needed packages.

This makes the default install script simpler: it doesn't need to check
NATIVE_DEPS or DOCS because both of those now override `install`
anyway, so the top-level `install` is really just the install for the
gcc-4.8 and osx builds; the docker builds and docs build override
install completely.

This switches the travis-ci build to build the docs in a dedicated
build rather than as part of the regular linux/osx builds.

Build sphinx docs (with -W) during travis-ci builds

Fix sphinx doc missing code blocks and warnings

The missing empty line after `.. code-block::` resulted in incorrectly
parsed restructuredtext (sphinx warnings) and the code blocks were not
generated in the html output.

The `exclude_patterns` change just silences the orphaned file warning.

[ci skip]