Remove outdated README info

[hugovk]

Remove "Requires Python 3.8+", support for 3.8 was dropped in #137.

These bits of texts are often forgotten, we can check pyproject.toml or see the README badges for the actual minimum required version:

[hugovk]

The pre-commit.ci failure is unrelated and a temporary error:

/pc/clone/Ee-_uc4HT52cFoUPstoiNQ/py_env-python3/bin/python -m pip install types-requests

WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f85905dc260>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/types-requests/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f85905dca40>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/types-requests/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f85905dcda0>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/types-requests/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f85905dd010>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/types-requests/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7f85905dd220>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/types-requests/
ERROR: Could not find a version that satisfies the requirement types-requests (from versions: none)
ERROR: No matching distribution found for types-requests