Update rsa to 4.8

[pyup-bot]

This PR updates rsa from 4.6 to 4.8.

Changelog

4.8

- Switch to [Poetry](https://python-poetry.org/) for dependency and release management.
- Chain exceptions using `raise new_exception from old_exception`
([157](https://github.com/sybrenstuvel/python-rsa/pull/157))
- Added marker file for PEP 561. This will allow type checking tools in dependent projects
to use type annotations from Python-RSA
([136](https://github.com/sybrenstuvel/python-rsa/pull/136)).

4.7.2

- Fix picking/unpickling issue introduced in 4.7
([173](https://github.com/sybrenstuvel/python-rsa/issues/173))

4.7.1

- Fix threading issue introduced in 4.7
([173](https://github.com/sybrenstuvel/python-rsa/issues/173))

4.7

- Fix [165](https://github.com/sybrenstuvel/python-rsa/issues/165):
CVE-2020-25658 - Bleichenbacher-style timing oracle in PKCS1 v1.5 decryption
code
- Add padding length check as described by PKCS1 v1.5 (Fixes
[164](https://github.com/sybrenstuvel/python-rsa/issues/164))
- Reuse of blinding factors to speed up blinding operations.
Fixes [162](https://github.com/sybrenstuvel/python-rsa/issues/162).
- Declare & test support for Python 3.9

Links

• PyPI: https://pypi.org/project/rsa
• Changelog: https://pyup.io/changelogs/rsa/
• Homepage: https://stuvel.eu/rsa

[pyup-bot]

Closing this in favor of #231