buildhistory-diff: suggest correct version of PythonGit

If buildhistory-diff can't import git module it throws error
message suggesting to install it.

Due to the move to Python 3 the suggested package name and
version became outdated.

Changed package name to python3-git and version to >= 0.3.4
as GitPython supports Python 3 starting from 0.3.4.

[YOCTO #9741]

(From OE-Core rev: 0632c3e69070e340218c1c1c835b2343cd143d8d)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/buildhistory-diff

@@ -14,7 +14,7 @@ from distutils.version import LooseVersion
 try:
     import git
 except ImportError:
-    print("Please install GitPython (python-git) 0.3.1 or later in order to use this script")
+    print("Please install GitPython (python3-git) 0.3.4 or later in order to use this script")
     sys.exit(1)
 
 def main():