Work on installation.

Try to make it easier and pex compatible

src/MANIFEST.in

@@ -1,3 +1,5 @@
 recursive-include msspec *.so
 recursive-include . SConstruct
+include setup_requirements.txt
 include requirements.txt
+include pip.freeze

src/pip.freeze

@@ -0,0 +1,56 @@
+alabaster==0.7.12
+ase==3.19.1
+Babel==2.8.0
+backcall==0.1.0
+certifi==2020.4.5.1
+chardet==3.0.4
+coverage==5.1
+cycler==0.10.0
+decorator==4.4.2
+docutils==0.16
+h5py==2.10.0
+idna==2.9
+imagesize==1.2.0
+ipython==7.13.0
+ipython-genutils==0.2.0
+jedi==0.17.0
+Jinja2==2.11.2
+lxml==4.5.0
+MarkupSafe==1.1.1
+matplotlib==2.1.1
+numpy==1.18.2
+packaging==20.3
+parso==0.7.0
+pex==2.1.11
+pexpect==4.8.0
+pickleshare==0.7.5
+Pillow==7.1.1
+Pint==0.11
+pip==20.0.2
+prompt-toolkit==3.0.5
+ptyprocess==0.6.0
+pycairo==1.19.1
+Pygments==2.6.1
+pyparsing==2.4.7
+python-dateutil==2.8.1
+pytz==2019.3
+requests==2.23.0
+scipy==1.4.1
+scons==3.1.2
+setuptools==46.1.3
+setuptools-scm==3.5.0
+six==1.14.0
+snowballstemmer==2.0.0
+spglib==1.14.1.post0
+Sphinx==3.0.1
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.4
+terminaltables==3.1.0
+traitlets==4.3.3
+urllib3==1.25.8
+wcwidth==0.1.9
+wheel==0.34.2

src/setup.py

@@ -24,7 +24,10 @@
 import sys
 sys.path.insert(0, "msspec")
 from setuptools import setup, find_packages
+try:
     from version import __version__
+except Exception as err:
+    __version__ = "8.8.8"
 
 
 with open('setup_requirements.txt', 'r') as fd:
@@ -33,12 +36,15 @@ with open('setup_requirements.txt', 'r') as fd:
 with open('requirements.txt', 'r') as fd:
     REQUIREMENTS = fd.read().strip().split('\n')
 
+with open('pip.freeze', 'r') as fd:
+    REQUIREMENTS = fd.read().strip().split('\n')
+
 if __name__ == "__main__":
     setup(name='msspec',
           version=__version__,
           include_package_data=True,
           packages=find_packages(include='msspec.*'),
-          setup_requires=SETUP_REQUIREMENTS,
+          #setup_requires=SETUP_REQUIREMENTS,
           install_requires=REQUIREMENTS,
 
           author='Didier Sébilleau, Sylvain Tricot',