Changes most fixed dependencies (#1491)

The previous requirements.txt had many fixed dependencies and I think
most of them don't need to be fixed.

Those fixed dependencies generate conflicts when mixing mythril with
other python/ethereum packages, notably brownie-eth.

requirements.txt

@@ -1,33 +1,33 @@
 coloredlogs>=10.0
 configparser>=3.5.0
 coverage
-py_ecc==1.6.0
-eth_abi==1.3.0
-eth-account>=0.1.0a2,<=0.3.0
+py_ecc
+eth_abi<3
+eth-account>=0.1.0a2,<0.6.0
 ethereum>=2.3.2
 ethereum-input-decoder>=0.2.2
 eth-hash>=0.3.1
 eth-keyfile>=0.5.1
-eth-keys>=0.2.0b3,<0.3.0
+eth-keys>=0.2.0b3,<0.4.0
 eth-rlp>=0.1.0
-eth-tester==0.1.0b32
+eth-tester
 eth-typing>=2.0.0
-eth-utils==1.9.0
+eth-utils<2
 jinja2>=2.9
 mock
-numpy==1.19.0
+numpy
 persistent>=4.2.0
 plyvel
 py-flags
-py-evm==0.3.0a13
-py-solc-x==1.0.0
+py-evm
+py-solc-x
 py-solc
 pytest>=3.6.0
 pytest-cov
 pytest_mock
-requests==2.25.0
+requests
 rlp>=1.0.1,<2.0.0
-semantic_version==2.8.5
+semantic_version
 transaction>=2.2.1
 z3-solver>=4.8.8.0
 pysha3