* Bump pyjwt and cryptography versions

Bump PyJWT to v2. No backward incompatible changes in the new version
affects oauthlib.

Bump minimum cryptography version to 3. PyJWT bumps cryptography as
well plus older versions don't support newer versions of openssl.

* Remove python 3.5 support; add python 3.8 support

- Python 3.5 has reached end-of-life.
- Add support for Python 3.8
- Move dist to bionic (xenial is EOL in April)
- Upgrade pip in travis.yml's `before_install`. this should install a wheel
  for pyca/cryptography
- Install rust for pypy builds. It is a requirement for cryptography as no
  pypy wheels exist.

Co-authored-by: Asif Saif Uddin <auvipy@gmail.com>

- Python 3.5 has reached end-of-life.
- Add support for Python 3.8
- Move dist to bionic (xenial is EOL in April)
- Upgrade pip in travis.yml's `before_install`. this should install a wheel
  for pyca/cryptography
- Install rust for pypy builds. It is a requirement for cryptography as no
  pypy wheels exist.

* Adding support for RSA-SHA256.

* Added support for HMAC-SHA512, RSA-SHA256 and RSA-SHA512 signature methods.

* Made version dependencies consistent.

* Updated OAuth1 signature tests.

* Fixed parsing of netloc/host. Deprecated old functions.

* Refactored and expanded tests to include signature validate.

* Update docs for HMAC-SHA512, RSA-SHA256 and RSA-SHA512 signature methods.

* Updated code comments in oauth1 signatures module.

* Updated changelog.

* Update docs/feature_matrix.rst

Co-Authored-By: Omer Katz <omer.drow@gmail.com>

* Used parenthesis instead of backslash to break lines.

* Fixed typo

Co-authored-by: Omer Katz <omer.drow@gmail.com>

Co-authored-by: Omer Katz <omer.drow@gmail.com>

* Update setup.py

* remove un needed python 2 codes

* remove un needed python 2 codes

* remove un needed python 2 codes

* Drop Python 3.4 support.

* Exclude Python 3.4 as a supported runtime.

* Update contributing.rst

* Update supported Python versions in README file.

Helps pip decide what version of the library to install.

https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

> If your project only runs on certain Python versions, setting the
> python_requires argument to the appropriate PEP 440 version specifier
> string will prevent pip from installing the project on other Python
> versions.

https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords

> python_requires
>
> A string corresponding to a version specifier (as defined in PEP 440)
> for the Python version, used to specify the Requires-Python defined in
> PEP 345.