Don't decode a unicode string when using git version

In #907, the git sha1 was added to the version string on
development builds.  In #2158, `encoding="utf-8"` was added
in `_minimal_ext_cmd`, which changed the output from a
bytestring to a regular string.  However, the original
code path still expects a bytestring and calls `.decode()` on it.
This fixes it in the way most obvious to me.

qiskit_aer/version.py

@@ -50,7 +50,7 @@ def git_version():
     # Determine if we're at main
     try:
         out = _minimal_ext_cmd(["git", "rev-parse", "HEAD"])
-        git_revision = out.strip().decode("ascii")
+        git_revision = out.strip()
     except OSError:
         git_revision = "Unknown"