Repository: Unbabel/COMET
Issue: fastBPE installation error
Root cause: Environment Incompatibility

## 🐛 Bug

When installing either using `pip install unbabel-comet` or directly with `pip install -r requirements.txt` I get an error `error: command 'x86_64-linux-gnu-gcc' failed with exit status 1`

`Running setup.py install for fastBPE ... error
    ERROR: Command errored out with exit status 1:
     command: /home/ubuntu/cometenv/bin/python3.7 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-eeqbscuj/fastbpe_45baab04cd16456fb32b018392790726/setup.py'"'"'; __file__='"'"'/tmp/pip-install-eeqbscuj/fastbpe_45baab04cd16456fb32b018392790726/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-record-hfrq684c/install-record.txt --single-version-externally-managed --compile --install-headers /home/ubuntu/cometenv/include/site/python3.7/fastBPE
         cwd: /tmp/pip-install-eeqbscuj/fastbpe_45baab04cd16456fb32b018392790726/
    Complete output (15 lines):
    running install
    running build
    running build_py
    package init file 'fastBPE/__init__.py' not found (or not a regular file)
    running build_ext
    building 'fastBPE' extension
    creating build
    creating build/temp.linux-x86_64-3.7
    creating build/temp.linux-x86_64-3.7/fastBPE
    x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -IfastBPE -I/usr/include/python3.7m -I/home/ubuntu/cometenv/include/python3.7m -c fastBPE/fastBPE.cpp -o build/temp.linux-x86_64-3.7/fastBPE/fastBPE.o -std=c++11 -Ofast -pthread
    fastBPE/fastBPE.cpp:28:10: fatal error: Python.h: No such file or directory
     #include "Python.h"
              ^~~~~~~~~~
    compilation terminated.
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /home/ubuntu/cometenv/bin/python3.7 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-eeqbscuj/fastbpe_45baab04cd16456fb32b018392790726/setup.py'"'"'; __file__='"'"'/tmp/pip-install-eeqbscuj/fastbpe_45baab04cd16456fb32b018392790726/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-record-hfrq684c/install-record.txt --single-version-externally-managed --compile --install-headers /home/ubuntu/cometenv/include/site/python3.7/fastBPE Check the logs for full command output.`

### Environment
(Ubuntu 18.04) Version 34.0
Python 3.7


Possibly the same issue as [Can't install in Ubuntu 20.04](https://github.com/glample/fastBPE/issues/45)

When inside the virtual environment run python and check if the gcc version is the correct one:

Working example on Ubuntu:
python
> Python 3.6.9 (default, Oct  8 2020, 12:12:24)
> **[GCC 8.4.0]** on linux
> Type "help", "copyright", "credits" or "license" for more information.

Working example on MacOS:
python
> Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 26 2018, 19:50:54)
> [**GCC 4.2.1** Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.

Bad configuration example on Ubuntu:
python
> Python 3.6.9 (default, Oct 28 2019, 16:14:01)
> **[GCC 8.3.0]** on linux
> Type "help", "copyright", "credits" or "license" for more information.