Tools: Use bitstring package compatible with supported Python versions

Bitstring>=4 supports Python 3.7+ only. See:
https://github.com/scott-griffiths/bitstring/releases/tag/bitstring-4.0.1

requirements.txt

@@ -29,7 +29,8 @@ kconfiglib==13.7.1
 
 # esptool requirements (see components/esptool_py/esptool/setup.py)
 reedsolo>=1.5.3,<=1.5.4
-bitstring>=3.1.6
+# bitstring 4 dropped support for Python 3.6 and older
+bitstring>=3.1.6,<4
 ecdsa>=0.16.0
 
 # espcoredump requirements