title: Getting error "No package 'libffi' found" when installing python-telegram-bot
body: <p>I tried to install python-telegram-bot on my raspberry pi but I got this problem:</p>
  
  <pre><code>Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-QLuRQr/cffi/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-1vjudm-record/install-record.txt --single-version-externally-managed --compile --user --prefix=:
      Package libffi was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libffi.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libffi' found
      Package libffi was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libffi.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libffi' found
      Package libffi was not found in the pkg-config search path.
      Perhaps you should add the directory containing `libffi.pc'
      to the PKG_CONFIG_PATH environment variable
      No package 'libffi' found
      ...
      arm-linux-gnueabihf-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-Ub3vap/python2.7-2.7.13=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DUSE__THREAD -DHAVE_SYNC_SYNCHRONIZE -I/usr/include/ffi -I/usr/include/libffi -I/usr/include/python2.7 -c c/_cffi_backend.c -o build/temp.linux-armv7l-2.7/c/_cffi_backend.o
      c/_cffi_backend.c:15:17: fatal error: ffi.h: No such file or directory
       #include &lt;ffi.h&gt;
                       ^
      compilation terminated.
      error: command 'arm-linux-gnueabihf-gcc' failed with exit status 1
  
      ----------------------------------------
  Command "/usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-QLuRQr/cffi/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-1vjudm-record/install-record.txt --single-version-externally-managed --compile --user --prefix=" failed with error code 1 in /tmp/pip-build-QLuRQr/cffi/
  </code></pre>
  
  <p>I tried this command but I got the same problem:</p>
  
  <pre><code>pip install python-telegram-bot==12.0.0b1 --upgrade 
  pip install python-telegram-bot --upgrade
  python -m pip install python-telegram-bot
  </code></pre>