Simplify the version mechanism.

No "setuptools-scm" anymore. The version is read by a subprocess
command or using package metadata or the VERSION file.
I still don't know if this is my last attempt to make the
version number reliable...

src/msspec/version.py

@@ -33,16 +33,18 @@ import subprocess
 
 PKGNAME = 'msspec'
 
+thisfile_path = os.path.abspath(__file__)
+thisfile_dir = os.path.dirname(thisfile_path)
+
 try:
     cmd = ["git describe|sed 's/-\([0-9]\+\)-.*/.dev\\1/g'"]
-    result = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, shell=True)
+    result = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, 
+            shell=True, cwd=thisfile_dir)
     __version__ = result.stdout.decode('utf-8').strip()
     if __version__ == "":
         raise
 except Exception as err:
     try:
-        thisfile_path = os.path.abspath(__file__)
-        thisfile_dir = os.path.dirname(thisfile_path)
         versionfile = os.path.join(thisfile_dir, "./VERSION")
         with open(versionfile, "r") as fd:
             __version__ = fd.readline().strip()