chore(deps): update dependency rsa to >=3.1.4,<4.2 (#524)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [rsa](https://stuvel.eu/rsa) ([source](https://togithub.com/sybrenstuvel/python-rsa)) | minor | `>=3.1.4,<4.1` -> `>=3.1.4,<4.2` |

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### Release Notes

<details>
<summary>sybrenstuvel/python-rsa</summary>

### [`v4.1`](https://togithub.com/sybrenstuvel/python-rsa/blob/master/CHANGELOG.md#Version-41---released-2020-06-10)

-   Added support for Python 3.8.
-   Dropped support for Python 2 and 3.4.
-   Added type annotations to the source code. This will make Python-RSA easier to use in
    your IDE, and allows better type checking.
-   Added static type checking via [MyPy](http://mypy-lang.org/).
-   Fix [#&#8203;129](https://togithub.com/sybrenstuvel/python-rsa/issues/129) Installing from source
    gives UnicodeDecodeError.
-   Switched to using [Poetry](https://poetry.eustace.io/) for package
    management.
-   Added support for SHA3 hashing: SHA3-256, SHA3-384, SHA3-512. This
    is natively supported by Python 3.6+ and supported via a third-party
    library on Python 3.5.
-   Choose blinding factor relatively prime to N. Thanks Christian Heimes for pointing this out.
-   Reject cyphertexts (when decrypting) and signatures (when verifying) that have
    been modified by prepending zero bytes. This resolves CVE-2020-13757. Thanks
    Adelapie for pointing this out.

</details>

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diff --git a/setup.py b/setup.py
index e02e9f9..2506d99 100644
--- a/setup.py
+++ b/setup.py
@@ -21,7 +21,7 @@
 DEPENDENCIES = (
     "cachetools>=2.0.0,<5.0",
     "pyasn1-modules>=0.2.1",
-    "rsa>=3.1.4,<4.1",
+    "rsa>=3.1.4,<5.0",
     "setuptools>=40.3.0",
     "six>=1.9.0",
 )