### Summary
Closes#1018. Enables `partition_email` and `partition_msg` to detect if
an email has PGP encrypted content. Based on the specification in [RFC
2015](https://www.ietf.org/rfc/rfc2015.txt). The test emails are based
on the example email in the spec. If PGP detected content is detected, a
warning is emitted and an empty set of lists is returned.
### Testing
```python
from unstructured.partition_email import partition_email
filename = "example-docs/eml/fake-encrypted.eml"
partition_email(filename=filename)
```
```python
from unstructured.partition_msg import partition_msg
filename = "example-docs/fake-encrypted.msg"
partition_msgl(filename=filename)
```
