Limit importlib_metadata in requirements (Qiskit#8828)

`importlib_metadata` 5.0 broke `stevedore`.  Normally we would set this
pin in `constraints.txt`, but since this is critical functionality and
we may need to release before `stevedore` is fixed, we fix the problem
at the requirements level, temporarily making `importlib_metadata` a
direct requirement.

requirements.txt

@@ -16,3 +16,9 @@ tweedledum>=1.1,<2.0; platform_machine != 'arm64' or sys_platform != 'darwin'
 # 1.1.1 has no wheel on M1 mac, which frequently causes problems for people.
 # Better just to forbid that in those cases.
 tweedledum>=1.1,<2.0,!=1.1.1; platform_machine == 'arm64' and sys_platform == 'darwin'
+
+# Hack around stevedore being broken by importlib_metadata 5.0; we need to pin
+# the requirements rather than the constraints if we need to cut a release
+# before stevedore is fixed.  `importlib_metadata` is not (currently) a direct
+# requirement of Terra.
+importlib_metadata<5.0; python_version<'3.8'