VPP-1508 Add support for environment markers.

Add the ability to specify a specific python library version
based on the interpreter/platform/etc.

Change-Id: I027acdf22ad839b5cff63b319f0aa100b0f336c8
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/Makefile

@@ -73,7 +73,7 @@ PYTHON_EXTRA_DEPENDS=
 endif
 
 PYTHON_VENV_PATH=$(VPP_PYTHON_PREFIX)/virtualenv
-PYTHON_DEPENDS=$(PYTHON_EXTRA_DEPENDS) psutil faulthandler six scapy==2.4.0 pexpect cryptography subprocess32 cffi syslog-rfc5424-parser
+PYTHON_DEPENDS=$(PYTHON_EXTRA_DEPENDS) -r requirements.txt
 SCAPY_SOURCE=$(shell find $(PYTHON_VENV_PATH) -name site-packages)
 BUILD_COV_DIR=$(BR)/test-cov
 

test/requirements.txt

@@ -0,0 +1,12 @@
+cffi                                    # MIT
+cryptography!=2.0                       # BSD/Apache-2.0
+faulthandler; python_version < '3.3' #  # BSD License (2 clause)
+flake8                                  # MIT
+ipaddress; python_version < '3.3'       # PSF
+pexpect                                 # ISC
+psutil                                  # BSD
+pycodestyle                             # MIT (Expat license)       https://pypi.org/project/pycodestyle/
+scapy==2.4.0; python_version >= '2.7' or python_version >= '3.4'    # GPL2  https://github.com/secdev/scapy/blob/master/LICENSE
+six                                     # MIT
+subprocess32                            # PSF
+syslog_rfc5424_parser                   # ISC