GitHub Actions: install python3-numpy

[traversaro]

This is now required since robotology/idyntree#726 .

Without this fix, the unstable CI is failing with the error:

2020-09-01T14:03:36.5315042Z CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
2020-09-01T14:03:36.5315306Z   Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) (found
2020-09-01T14:03:36.5315430Z   version "3.8.2")
2020-09-01T14:03:36.5315536Z Call Stack (most recent call first):
2020-09-01T14:03:36.5315828Z   /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)
2020-09-01T14:03:36.5316211Z   /usr/share/cmake-3.16/Modules/FindPython/Support.cmake:2214 (find_package_handle_standard_args)
2020-09-01T14:03:36.5316487Z   /usr/share/cmake-3.16/Modules/FindPython3.cmake:300 (include)
2020-09-01T14:03:36.5316611Z   bindings/python/CMakeLists.txt:3 (find_package)

[traversaro]

On Debian Stable, apparently some other packages are necessary to install:

2020-09-01T16:04:56.9017901Z CMake Error at /usr/share/cmake-3.13/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
2020-09-01T16:04:56.9018061Z   Could NOT find Python3 (missing: NumPy) (found version "3.7.3")
2020-09-01T16:04:56.9018193Z Call Stack (most recent call first):
2020-09-01T16:04:56.9018521Z   /usr/share/cmake-3.13/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
2020-09-01T16:04:56.9018884Z   /usr/share/cmake-3.13/Modules/FindPython/Support.cmake:1130 (find_package_handle_standard_args)
2020-09-01T16:04:56.9019211Z   /usr/share/cmake-3.13/Modules/FindPython3.cmake:163 (include)
2020-09-01T16:04:56.9019348Z   bindings/python/CMakeLists.txt:3 (find_package)

[traversaro]

On Debian Stable, apparently some other packages are necessary to install:

2020-09-01T16:04:56.9017901Z CMake Error at /usr/share/cmake-3.13/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
2020-09-01T16:04:56.9018061Z   Could NOT find Python3 (missing: NumPy) (found version "3.7.3")
2020-09-01T16:04:56.9018193Z Call Stack (most recent call first):
2020-09-01T16:04:56.9018521Z   /usr/share/cmake-3.13/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
2020-09-01T16:04:56.9018884Z   /usr/share/cmake-3.13/Modules/FindPython/Support.cmake:1130 (find_package_handle_standard_args)
2020-09-01T16:04:56.9019211Z   /usr/share/cmake-3.13/Modules/FindPython3.cmake:163 (include)
2020-09-01T16:04:56.9019348Z   bindings/python/CMakeLists.txt:3 (find_package)

Apparently the NumPy component of FindPython3 is just supported by CMake 3.14 (see https://cmake.org/cmake/help/v3.14/module/FindPython3.html), but Debian Buster by default has CMake 3.13 .

[traversaro]

As this PR is still an improvement even if it does not fix completely the problem, I will merge it.