Fix RSA keyfile open mode (rb) for python3 (#141)

[joeleong]

Proposed fix for #141

I only really tested it with the m2crypto signer, but I assumed it would be the same for sign_pythonrsa.py

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[bibz]

Experiencing the same issue, I validated the fix on my side with Python RSA 4.0 and Python 3.6:

$ cd $(mktemp -d)
$ git init .
$ git fetch https://github.com/joeleong/python-adb.git 3338e6baa4ce37523e5859094bf7b758b926c2e3
$ git checkout FETCH_HEAD
$ virtualenv -p python 3 venv
$ . ./venv/bin/activate
$ pip install rsa==4.0
$ pip install -e .

I followed the steps to reproduce as mentioned in #141 :

1. Revoke USB debugging authorisation
2. Run pyadb shell ls
3. Acknowledge that the device now asks to allow access from the host

[joeleong]

I signed it!

[googlebot]

CLAs look good, thanks!

[greyltc]

What needs to be done to get this merged? The bug in master that this PR fixes seems to make the project useless under python 3.